When a subscriber base grows exponentially, that is a success. But rapid growth has a downside: the tools that worked yesterday can slow down scaling.

This is exactly the challenge faced by the Prometheus educational platform. When the total number of subscribers reached 3 million, it became clear that scaling further would require not only an ESP, but also advanced segmentation and detailed analytics.

In this case study, we describe how Prometheus migrated the core of its database, more than 1.5 million active contacts, from the Sendy platform to the Yespo CDP in just 8 weeks.

It is the story of how to move a large-scale project to a new platform quickly while keeping the domain reputation flawless and audience engagement high.

Database	1.5+ million users
Communication channel	Email
Goals	• Migrate email marketing from an ESP to the Yespo CDP. • Warm up the email sending domain while keeping its reputation at High.
Results	• Reached a domain sending volume of 1,358,734 contacts. • Kept the reputation at High, both during and after the warm-up.
Duration	8 weeks
Resources	• A marketer from Prometheus • A marketer from the Yespo agency

About the Project

Prometheus is a large online professional development platform, founded in 2014. The project specializes in creating professional programs and localizing the best educational materials from around the world, making the expertise of leading instructors available to everyone. Over 11 years, the platform has helped its partners systematize their experience and has also provided educational support for the most important state reforms.

From ESP to CDP Capabilities: How and Why Prometheus Changed Platforms

Moving to a new platform carries risks: new technical settings, possible delivery failures, and reputational threats to the domain. Prometheus took this step deliberately: once the total database grew to 3 million subscribers, the existing toolset no longer matched the team's ambitions and pace of development.

Here is how that decision took shape, and why the platform chose Yespo.

Why Prometheus Outgrew Its ESP

For over 5 years, the company used the Sendy ESP for mass email campaigns.

It was a reliable tool while the project was taking shape, but rapid growth and scaling brought new communication requirements. The business had outgrown the mailing service's capabilities, and the need for automation became critical.

The main factors that led to the search for a new solution and the migration:

• Manual management. The mailing system was not deeply integrated with the learning platform. The team had to export and import data manually, which took time and increased the risk of errors.
• Lack of data for segmentation. Because of technical limitations, mailings went to broad segments ("to everyone"), although the project needed to reach different audiences precisely and simultaneously.
• Limited testing. The ESP's capabilities restricted marketing flexibility: the team could not fully experiment with content, test hypotheses, or implement automated trigger workflows that react to user actions in real time.

Why They Chose the Yespo CDP

The "Parallel Start" Strategy: How to Migrate Without Communication Pauses or Revenue Loss

The biggest fear of any brand during a migration is a communication vacuum: a pause in mailings can lower engagement and cause direct losses. To eliminate these risks, the Prometheus team chose a gradual transition strategy.

During the 8-week migration and warm-up, the project ran two services in parallel.

Advantages of working in parallel:

• Stability. While technical setup and warm-up were under way in Yespo, critical mailings were sent through the old ESP.
• Safe data transfer. Because both services ran in parallel, automatic transfer of contacts from registrations could be set up without the risk of losing new users' data.
• Cost optimization. Paying for two platforms at once is an additional burden on the budget. However, with a database of 3 million contacts, it helped avoid a potential loss of audience loyalty from interrupted learning workflows.

The Prometheus team carried out the main stage of the migration on its own, relying on the Yespo documentation.

Prometheus's Migration Plan to Yespo

1. Contact database transfer. The 1.5 million most active contacts were exported and then imported into the CDP, so that the warm-up would focus on the most loyal customers. The remaining contacts were left on the old ESP for later reactivation.
2. Verification of the domains used in Sendy, since they had already been warmed up for large sending volumes and had a good reputation and recognition.
3. Content adaptation and design. Transferring templates to Yespo and adjusting the layout.
4. Automatic data transfer from the website. Setting up contact transfers from site registrations.
5. Sender warm-up. Gradually increasing sending volumes to reach the permitted limits and switch to sending with Prometheus's own digital signatures.

Tools Used During the First Year of Cooperation

During the first year, the Prometheus team used the basic capabilities of the CDP, focusing on automating data collection and analytics.

What they used:

• Integration with the learning platform: set up automatic transfer of contacts from registrations into the corresponding groups based on users' interests.
• A/B testing module: to test content hypotheses (for example, the effect of FOMO on Open Rate).
• Analytics dashboard and Postmaster Tools: for daily monitoring of domain reputation as seen by Google, and for tracking delivery errors (Hard/Soft Bounce).
• Yespo's flexible editor: for creating responsive emails independently, without involving developers. This made it possible to move away from the old system's standard layouts and implement complex technical parameters: dark theme support, correct display on mobile devices, and a strict weight limit of 100 KB, which is critical for stable delivery to Gmail.

How to Warm Up a Domain for 1.5 Million Contacts: A Step-by-Step Algorithm for Prometheus

The Prometheus team already had its own know-how, but to minimize risks and strengthen the strategy, it turned to the Yespo agency for support. The collaboration combined Prometheus's deep knowledge of its audience with Yespo's experience working with large databases. This made it possible to adapt the warm-up pace to the actual reaction of mail providers and to Yespo's sending limits for specific providers.

A Yespo marketer helped adapt the warm-up plan to the database size and monitored results at each stage.

Stage 1. Segment Preparation

The database of 1.5 million contacts was divided by two criteria:

• Segmentation by activity. The team identified those who actively engaged with the previous mailing: 661 contacts across different domains. High engagement helped create an ideal initial reputation profile for the domain.
• Distribution by domain type. Next, the database was divided into Gmail users, who made up about 79% of the audience (1.23 million), and everyone else. The number of other domains was small, and warming them up did not require lengthy, systematic effort, so we combined them into one segment: 21%, or 331 thousand. Splitting by domain type made it possible to build a flexible sending schedule that accounted for each mail provider's limits.

Stage 2. Developing a Content Strategy for the Warm-Up

Once the segments are formed, the key task is choosing what the messages will say. The priority is email opens and link clicks.

Content criteria:

• Maximum relevance: topics of interest to a wide range of platform users.
• Ease of interaction: clear, appealing elements that people want to click.
• No aggressive marketing: no pushy discounts or direct course sales.
• Focus on value: an emphasis on direct benefit and social significance for the recipient.

For the warm-up campaign, a holiday occasion was combined with gamification. For Prometheus's 10th anniversary, the team prepared a prize giveaway that motivated users to engage with the email.

To carry out the warm-up, they chose the giveaway campaign and split it into two emails.

1. In the first email, Prometheus announced the giveaway and invited subscribers to take part. The prizes matched the educational focus of the project: certificates for paid Prometheus courses, books on relevant topics, and the chance to ask lecturers questions in a closed Q&A session.
2. The second email contained the giveaway results. Since subscribers were waiting for the winners to be announced, it had the potential to get high open rates and to help build the new sender's positive reputation with mail services faster.

Stage 3. Preparing the Emails: Copy, Design, and Layout

To warm up a large database, it is important to prepare emails that are technically sound. Deliverability is affected by the subject line, email structure, text-to-image ratio, HTML weight, responsiveness, and even the presence of alt text.

So before launch, the team checked each email against a basic checklist:

• Relevance. The subject line matched the content inside, with no clickbait or inflated expectations.
• Text-to-image balance. The email was not a single large image: the target was at least 60% text content and up to 40% visual content. This helps mail services "read" the email better and reduces the risk of landing in spam.
• Accessibility. Alt text was added to images, and the unsubscribe link was kept visible and clear.
• Technical hygiene. The email's HTML was kept under 100 KB so Gmail wouldn't clip content.
• Responsiveness. Emails were checked on desktop, on smartphones, and in dark theme to make sure they display correctly for all users.

A/B Testing: Which Subject Line Worked Better?

At one of the first stages of the warm-up, the team tested subject lines on a smaller segment of the database. They tested what works better for opens: FOMO or direct benefit. The winning subject line was used in the following waves with larger sending volumes.

• Variant A: 🏆 Hurry up and claim your prize! Big giveaway for Prometheus's 10th anniversary
• Variant B: 🔥 Your chance to win a course: celebrating Prometheus's 10th anniversary!

The test ran for 1 month, which is how long it took to reach the sample size needed to evaluate the results correctly.

Test results:

Subject line variant	Open Rate
🏆 Hurry up and claim your prize! Big giveaway for Prometheus's 10th anniversary 🔥	30.9%
🔥 Your chance to win a course: celebrating Prometheus's 10th anniversary!	27%

The first variant won, with an OR of 30.9% versus 27% for the second. This confirmed that calls to action containing an element of urgency ("hurry up") drive significantly higher engagement.

The biggest spike in activity came from the email announcing the winners. Thanks to the high anticipation of the results, it achieved an Open Rate of 68.6% and a CTR of 39.7%.

Engagement results from the giveaway results email:

Email received by	12,505 giveaway participants
Email Open Rate	68.6%
Email CR	27.2%
CTR	39.7%

Stage 4. Warm-Up Schedule: From the First 600 to 1.3 Million Emails

The main goal of the schedule is to put a smooth load on mail services, without sudden spikes in activity that could be perceived as spam. For Prometheus, the warm-up lasted 8 weeks and was based on the principle of "controlled speed".

Important. Increase email volume gradually. On the first day of warming up a new domain, you can send no more than 1,000 emails; otherwise, the mailing may be flagged by the mail server as unwanted activity.

Prometheus began carefully, sending only 585 messages on the first day to the active segment of those who had engaged with the previous mailing.

They started with ten sends per minute and then followed the prepared schedule:

1. On the first day, they used the segment of active audience members who had read and clicked through the warm-up email prepared by the Prometheus team:
   • Gmail domain – 383 contacts;
   • other domains – 202 contacts.

First day of the warm-up

Domain	Sending volume
gmail	383 contacts
others	202 contacts

1. Next, they continued with the previously prepared segment of active audience members, those who had read previous emails: 54,189 contacts. It was also split into subgroups by domain:
   • the largest (Gmail) – 37,802;
   • contacts from other domains were combined into one group – 13,984.
2. The subgroups were then split further to increase sending volumes gradually by the number of messages sent.

Sending volumes on the second day of the warm-up

Domain	Number of contacts
yahoo.com	992
lll.kpi.ua	613
gmail	567
oa.edu.ua	526
icloud.com	529
other domains	504
ukr.net	463
outlook.com	272
hotmail.com	167
meta.ua	103
i.ua	117
bigmir.net	25
Total sending volume for the day	4,878

The schedule for the following days of the domain warm-up was built the same way: volumes grew daily, guided by the providers' reaction.

You can find detailed instructions for domain warm-up in the Yespo knowledge base.

Example of the warm-up schedule for the Gmail domain over the whole period:

Daily sending limit for the Gmail domain, for the segment of those who opened emails

Day	Sending volume
1	383
2	567
3	1,116
4	2,166
5	4,071
6	6,270
7	9,080
8	14,505
9	20,894
10	30,195
11	35,908
12	40,261
13	9,536
14	9,445
15	10,324
16	11,178
17	11,925
18	12,671
19	13,414
20	14,938
21	16,478
22	17,772
23	19,990
24	20,989
25	22,040
26	23,730
27	23,736
28	25,121
29	27,168
30	28,786
31	30,752
32	55,218
33	80,030
34	120,387
35	153,618
36	198,531

Flexibility Instead of a Rigid Plan: The Day 13 Case

Having a schedule does not mean it cannot be changed. On the contrary, the value of Prometheus's experience lies in reacting in time to signals from mail services.

On day 12, sending volumes peaked at 40,261 emails. However, Postmaster Tools recorded a decline in metrics, so the team immediately adjusted the plan for day 13:

1. The volume of sends to the Gmail domain was reduced to 9,536 emails (setting a safe limit of up to 10,000 per day).
2. For the next several days, the volume was increased very carefully (about 10–12 thousand) until the reputation in Postmaster stabilized.
3. Only after the reputation reached "High" did the team return to large-scale growth, reaching 198,531 sends on day 36.

Technical monitoring. During the warm-up, the team constantly tracked key metrics so it could adjust sending volumes in time and avoid risking the sender's reputation:

1. Google Postmaster Tools – they checked the domain and IP reputation, the complaint rate, and technical errors. This helped them understand how Google evaluates the sender's new activity.
2. Yespo analytics – they monitored unsubscribes, delivery errors (Hard Bounce / Soft Bounce), and Open Rate. This made it possible to assess the quality of segments and maintain database hygiene at every stage of the warm-up.

Warm-Up Results and Reputation Status

Prometheus completed the domain warm-up with metrics that confirmed the chosen strategy was right. Over 8 weeks, the team built a reliable foundation for the further development of its learning ecosystem.

Flawless Reputation and Reach

The main technical achievement was successfully warming up the domain to a level of 1.35 million contacts. According to Google Postmaster Tools, the sender's reputation stayed at the highest level throughout the entire period.

This indicates full trust from mail services: emails reach the inbox without obstacles, and the number of spam complaints is kept to a minimum.

Engagement at the Peak: Analytics for the Last Days

The metrics for the last 5 days of the warm-up (using the Gmail segment as an example) show stable audience interest even with a sharp increase in sending volumes.

Day	Messages sent	Email OR	Email CR	CTR
32	55,218	28.6%	1.1%	3.9%
33	80,030	28.6%	1.1%	3.7%
34	120,317	28.6%	1.1%	3.8%
35	149,209	28.3%	1.1%	3.8%
36	198,414	28.2%	1.0%	3.7%

After the warm-up was completed on December 16, 2024 (the first send was on November 5, 2024), the database covered by the warm-up was 1,358,734 contacts.

The email announcing the giveaway winners showed the highest engagement. By skillfully using audience anticipation, it achieved a record Open Rate of 68.6% and a CTR of 39.7%.

From Intuition to Numbers: A Strategic Change of Approach

The move to the Yespo CDP allowed Prometheus to overcome the limitations of an ESP and reach a new level of data-driven decision-making. The project gained tools for detailed result tracking, which immediately improved the quality of its marketing decisions.

Plans for the Future

Prometheus is now moving on to the next strategic stage: deep technical integration. This will transform the email channel from a mass-mailing tool into a system of personalized communication that adapts to each user's behavior in real time.

With the capabilities of the Yespo CDP, the team plans to increase marketing effectiveness through deeper segmentation and the launch of automated workflows that react to user actions:

• Welcome series: for effective onboarding of new subscribers and introducing them to the brand's values.
• Abandoned landing page view: automatically bringing interested visitors back to course pages.
• Unfinished learning: personalized triggers that encourage students to continue their educational path.
• Reactivation: systematic work with inactive contacts to restore their engagement.

Combining mass mailings with automated trigger workflows will allow Prometheus to accompany each user on their way to new knowledge.

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