Should I continue sending from my old ESP while warming up a new IP address?
Michael Ko
Co-founder & CEO, Suped
Published 18 Apr 2025
Updated 19 Aug 2025
6 min read
Migrating to a new Email Service Provider (ESP) is a significant undertaking that requires careful planning, particularly when it comes to warming up a new IP address. A crucial question often arises during this transition: should I continue sending emails from my old ESP while simultaneously warming up my new IP? It's a valid concern, as businesses want to maintain their email program's effectiveness and avoid any drop in revenue or engagement.
The short answer is, yes, it's generally advisable to continue sending from your old ESP during the initial phases of warming up a new IP address. This overlapping approach allows you to gradually shift your email volume, ensuring a smoother transition and protecting your sender reputation. Stopping all sending abruptly can be detrimental to your existing email program and create challenges for your new IP.
Why overlapping sending benefits your reputation
IP warming is the process of building a positive sender reputation for a new IP address by gradually increasing your email sending volume. Internet Service Providers (ISPs) like Microsoft and Gmail monitor sending patterns from new IPs very closely. A sudden surge in volume from a cold IP can trigger spam filters, leading to emails being rejected, sent to the spam folder, or even the IP being added to a blocklist (or blacklist).
Maintaining a consistent sending presence through your old ESP ensures that your existing sender reputation remains stable while you establish trust with the new IP. If you halt all sending, your domain's reputation could degrade due to inactivity, making it harder for the new IP to gain traction. The goal is a gradual, controlled shift, not an abrupt stop and restart.
Think of it like easing into a new driving lane on a busy highway. You don't suddenly swerve, but rather merge smoothly while maintaining your speed in your current lane. This minimizes disruption and risk. The same principle applies to email migration and IP warming, where a sudden change can lead to deliverability issues.
The gradual approach
Gradually increasing send volume allows ISPs to observe consistent positive engagement, which builds your new IP's reputation. This is critical for avoiding spam folders and blocklists (or blacklists).
Risk of abrupt changes
Sudden changes in sending volume or a complete halt followed by a large burst from a new IP can be flagged as suspicious behavior by ISPs, negatively impacting your sender reputation.
Segmenting your audience for a smooth transition
The most effective way to manage dual sending is through careful audience segmentation. You should always begin your IP warming process by sending to your most engaged subscribers on the new IP. These are the recipients most likely to open your emails, click on links, and generally interact positively, which sends strong positive signals to ISPs.
As your new IP builds a positive reputation with these highly engaged contacts, you can gradually increase the volume and expand to less engaged segments. During this time, your old ESP can continue to handle the bulk of your less engaged audiences or specific email types (like promotional one-offs or transactional emails) until the new IP is fully warmed.
This phased approach minimizes the risk of deliverability issues, ensuring that your core audience continues to receive your messages without interruption. It also allows you to maintain revenue streams from other email campaigns while the new IP gains credibility.
Old ESP sending
Audience: Less engaged segments or high-volume promotional sends.
Email types: Transactional emails, daily newsletters, or one-off marketing campaigns.
Goal: Maintain existing revenue and sender reputation during migration.
New ESP warming
Audience: Most engaged, active, and recent subscribers.
Email types: Regular, high-engagement content like weekly digests or transactional confirmations.
Goal: Build a strong, positive sender reputation for the new IP.
Avoiding common pitfalls during dual sending
When simultaneously sending from two ESPs, it's crucial to avoid sending duplicate emails to the same recipient. This can quickly lead to high complaint rates and damage your sender reputation, potentially landing both your old and new IPs on a blocklist (or blacklist). Implement robust segmentation and suppression rules to ensure no overlap in your sending lists.
Consider your domain configuration as well. While the IP addresses are changing, your sending domain remains a constant identifier. Ensure your DNS records (SPF, DKIM, DMARC) are correctly configured for both ESPs during the transition, if applicable. A misconfiguration can lead to authentication failures and affect your deliverability.
Finally, be transparent with your subscribers if necessary. If your from address or sending domain needs to change, or if they might notice a slight difference in email headers, a brief announcement can prevent confusion or marked spam reports. This proactive communication can foster trust and prevent unnecessary damage to your overall email deliverability.
Aspect
Key action during transition
Why it matters
Audience segmentation
Prioritize engaged subscribers on the new IP first.
Builds trust with ISPs efficiently.
Duplicate suppression
Ensure no recipient receives emails from both ESPs.
Successful IP warming isn't a set-it-and-forget-it process. It requires constant vigilance and adaptation. Closely monitor your deliverability metrics, including inbox placement rates, open rates, click-through rates, bounce rates, and, most importantly, complaint rates. Tools like ESP-provided analytics and Google Postmaster Tools are invaluable for this.
If you notice an increase in bounces or complaints on your new IP, it's a signal to slow down your sending volume and re-evaluate your strategy. Conversely, if engagement is consistently strong and deliverability is high, you can cautiously accelerate your warm-up schedule. The process typically takes anywhere from a few weeks to several months, depending on your list size and sending frequency.
Only when your new IP has demonstrated consistent, strong performance and positive reputation with key ISPs should you fully transition all your email traffic away from your old ESP. This methodical approach minimizes disruption and sets your new sending infrastructure up for long-term success, avoiding issues like emails going to spam.
Positive outcomes
A properly warmed IP address leads to improved inbox placement, higher engagement rates, and a stronger sender reputation. This translates directly into better email campaign performance and sustained revenue.
Views from the trenches
Best practices
Maintain a consistent sending volume from your old ESP to avoid a sudden drop in reputation.
Segment your audience, sending only the most engaged recipients to the new IP initially.
Implement a clear ramp-up schedule for the new IP, gradually increasing volume and frequency.
Prioritize sending emails with high anticipated engagement (e.g., transactional, welcome series) via the new IP.
Common pitfalls
Abruptly stopping all sending from the old ESP, which can harm your domain's reputation.
Sending duplicate emails to the same subscribers from both ESPs, leading to high complaints.
Not monitoring deliverability metrics closely enough on the new IP, missing warning signs.
Trying to warm up too quickly, triggering spam filters and blocklists (or blacklists).
Expert tips
Consider informing your subscribers about the migration to a new sending platform if 'from' address changes.
Ensure your DNS records (SPF, DKIM, DMARC) are correctly configured for both ESPs during the transition.
Be patient, as IP warming takes time, and rushing it can lead to long-term deliverability issues.
Use a dedicated subdomain for your new ESP to manage reputation independently and track bounces.
Expert view
Expert from Email Geeks says overlapping migration is generally recommended whenever possible for a smoother transition.
2020-02-12 - Email Geeks
Expert view
Expert from Email Geeks advises starting with the most engaged recipients on new IPs and continuing to send to less engaged ones from old IPs, ensuring recipients don't forget about you.
2020-02-12 - Email Geeks
The strategic approach to a seamless ESP migration
Deciding whether to continue sending from your old ESP during a new IP warm-up is a critical decision in any ESP migration. The consensus leans strongly towards an overlapping, gradual transition rather than an abrupt cutoff. This strategy helps preserve your established sender reputation and mitigates the risks associated with starting fresh.
By carefully segmenting your audience and prioritizing your most engaged subscribers for the new IP, you can effectively build trust with ISPs. Simultaneously, your old ESP can manage other sending volumes, ensuring that you don't miss out on important revenue opportunities or lose connection with your wider audience. This dual-platform approach offers a strategic pathway to a successful migration.
Remember, the key to a smooth IP warming is consistency, careful monitoring, and a willingness to adjust your strategy based on real-time deliverability data. Investing this time upfront will pay dividends in sustained inbox placement and overall email program health down the line.
Gmail monitor sending patterns from new IPs very closely. A sudden surge in volume from a cold IP can trigger spam filters, leading to emails being rejected, sent to the spam folder, or even the IP being added to a blocklist (or blacklist).
