Is IP warm-up always necessary after an ESP server migration?

An IP warm-up is essential after an ESP server migration if you are assigned new IP addresses. New IPs have no prior sending history, so mailbox providers view them with caution. Gradually increasing volume and ensuring positive engagement helps build a trusted sending reputation for the new IPs, preventing your emails from being sent to spam or blocklisted (blacklisted).

How do I choose which contacts to send to during the initial warm-up phase?

Start by sending to your most engaged segments first (those who recently opened or clicked your emails). Their positive interactions help establish a good reputation for your new IP. Gradually expand your audience, introducing less engaged segments only when your IP's reputation is strong.

How long does an IP warm-up typically take?

A standard warm-up period can range from 2 to 6 weeks, depending on your sending volume and list quality. Higher volumes and less engaged lists may require a longer warm-up. Consistency is more important than speed; do not rush the process. Read more about warm-up schedules .

What metrics should I monitor during IP warm-up, and what do they indicate?

Monitor your open rates, click-through rates, bounce rates, and spam complaint rates daily using your ESP's analytics and postmaster tools (e.g., Google Postmaster Tools). A sudden increase in bounces or complaints indicates you're sending too much too soon, or to the wrong audience.

Does my domain reputation carry over to a new IP address after migration?

Yes, even if your domain reputation is strong, a new IP still needs to establish its own trust. Mailbox providers assess both domain and IP reputation. While a good domain reputation helps, a cold, unwarmed IP can still lead to deliverability issues.

What is the best IP warm-up strategy after an ESP server migration? - Sender reputation - Email deliverability - Knowledge base

Migrating to a new email service provider (ESP) or experiencing a server migration often means sending from new IP addresses. This isn't just a technical detail; it directly impacts your email deliverability. Mailbox providers, like Twilio SendGrid or Amazon SES, assess the sending reputation of an IP address. A new, unwarmed IP, lacks this established reputation and can trigger spam filters, leading to your emails landing in the junk folder or being blocklisted (blacklisted) outright. This is why a strategic IP warm-up is non-negotiable.

The goal is to gradually increase your sending volume on the new IP, building trust with mailbox providers over time. This process helps them recognize your sending patterns as legitimate and desired by recipients. Without a proper warm-up, even highly engaged subscribers might miss your important communications, impacting everything from marketing campaigns to critical transactional messages.

Understanding the impact of server migration on IPs

A server migration by your ESP often necessitates new IP addresses, even if your domain remains the same. This is typically due to shifts in physical data center locations or updates to their underlying sending infrastructure. While it might seem counterintuitive to change IPs after a migration, especially if you've built a strong reputation, sometimes it is unavoidable.

The core issue is that each IP address has its own sending history and reputation. When you move to a new IP, it's essentially a blank slate. Mailbox providers see it as an unknown sender, and their default stance is often cautious, bordering on suspicious, to prevent spam. This is why the warm-up process is critical, regardless of your past reputation on old IPs.

For your scenario, with daily sends of 30-50k and a large weekly send of 700k, the challenge is scaling back up to that volume without triggering spam filters. A sudden jump in volume from a new IP is a red flag for mailbox providers. They look for consistent, gradual increases in sending volume coupled with positive recipient engagement.

The risk of ignoring IP warm-up

Failing to implement a proper IP warm-up strategy after an ESP server migration can have severe consequences for your email program:

Spam folder placement: Emails sent from a cold IP are often flagged as suspicious and diverted to the spam folder, drastically reducing inbox placement and visibility.
Blocklisting: Rapid increases in volume or poor engagement can lead to your new IP being added to a blocklist (or blacklist), preventing emails from reaching recipients entirely. Learn more about email blocklists.
Reputation damage: A poor warm-up can establish a negative sender reputation for your new IP, which is very difficult and time-consuming to reverse, hindering future deliverability.
Lost revenue: If emails aren't reaching the inbox, you lose opportunities for engagement, conversions, and revenue, especially with time-sensitive content.

Crafting your IP warm-up schedule

The key to a successful IP warm-up is a methodical and gradual increase in volume. This isn't a race, but a marathon where consistency beats speed. You should always start with your most engaged subscribers, as their positive interactions (opens, clicks) send strong signals of legitimacy to mailbox providers.

For your significant weekly send of 700k emails, you'll need a tailored approach. Instead of halting this critical communication, you can gradually shift portions of this audience to the new IP. Begin by sending smaller segments of your weekly list, focusing on the most active subscribers, and steadily expand the segment size each week until the entire list is migrated.

It's also beneficial to spread your sends throughout the day rather than sending large bursts at once. This mimics natural sending behavior and helps avoid overwhelming mailbox providers. Remember that the goal is to build a positive sending history.

Week	Daily Volume (Approx.)	Audience Focus	Key Metrics to Monitor
Week 1	10,000 - 20,000	Most engaged subscribers (opened/clicked in last 30-60 days)	High open rates (>20%), low bounce/complaint rates (<0.1%)
Week 2	20,000 - 50,000	Engaged subscribers (opened/clicked in last 90-120 days) + a portion of weekly list	Maintain high engagement, monitor for any increase in negative metrics
Week 3	50,000 - 150,000	More of weekly list, gradually introducing less recent engagement	Ensure engagement scales with volume, address any spikes in complaints
Week 4+	150,000+ (as reputation allows)	Continue expanding to full list, mix in less engaged segments if performing well	Aim for consistent deliverability across all segments, monitor for blocklists (blacklists)

Audience segmentation for successful warming

Your intuition to segment audiences is spot on. For time-sensitive content, it’s crucial to minimize risk. By prioritizing your most engaged subscribers, you are sending a positive signal to mailbox providers. These are the individuals most likely to open, click, and generally interact positively with your emails, which is exactly what you need to build trust for a new IP.

When you introduce less engaged segments, it's wise to combine them with a portion of your highly engaged audience. This approach helps dilute any potential negative impact from lower engagement rates in the less active group. The key is to avoid large, sudden sends to unengaged or dormant segments, which can quickly lead to spam complaints and damage your new IP's reputation.

Carefully manage the content and frequency, especially during the initial weeks. Even with an engaged audience, over-sending or irrelevant content can lead to unsubscribes or spam complaints, hindering your warm-up progress. For additional insights on email volume scaling, consider best practices for your specific sending needs.

Engaged segments

Prioritize: Start your warm-up by sending to subscribers who have opened or clicked your emails most recently (e.g., last 30-60 days). They are your strongest advocates.
Benefits: High open rates and low complaint rates from these users signal to Microsoft Gmail and other providers that your new IP is trustworthy.
Content Strategy: Continue sending your most valuable, highly anticipated content to this group. For your time-sensitive online events, these are the ideal recipients.

Less engaged segments

Introduce gradually: Once your IP shows strong positive metrics, begin to slowly introduce segments of subscribers who haven't engaged recently (e.g., 90-180 days or more). Split your larger weekly sends across these groups.
Mitigate risk: If their engagement is lower or complaints rise, you can pause or reduce volume to these segments without jeopardizing your overall warm-up.
Re-engagement campaigns: Consider a dedicated re-engagement campaign for very old segments on the old IP first, or a smaller, hyper-targeted campaign on the new IP, before fully integrating them.

Monitoring and maintaining reputation

Throughout the warm-up process, vigilance is key. You need to constantly monitor your email metrics, including open rates, click-through rates, bounce rates, and crucially, complaint rates. An unexpected spike in bounces or complaints is a clear sign to slow down your sending volume until these metrics stabilize. This proactive approach helps prevent your IP from being added to a blocklist (or blacklist).

Leverage any available postmaster tools from major mailbox providers like Google Postmaster Tools or Yahoo Postmaster. These tools provide valuable insights into your sending reputation, spam complaint rates, and deliverability issues, helping you make informed decisions about your sending volume. You can also sign up for feedback loops (FBLs) offered by various providers. FBLs notify you when recipients mark your emails as spam, allowing you to promptly remove those addresses from your list.

Regularly checking your IP against common public blocklists (blacklists) is also a critical step. If your IP appears on a blocklist (blacklist), it indicates a significant problem that needs immediate attention. You would then need to work on delisting your IP and reassessing your sending practices. Consistent positive engagement and adherence to best practices are the best defenses against being blocklisted (blacklisted).

Views from the trenches

Best practices

Start with your most engaged subscribers to build a strong initial reputation on new IPs.

Gradually increase email volume based on positive engagement metrics.

Segment your audience effectively, prioritizing those with recent interactions.

Monitor key metrics like open rates, click rates, and complaint rates daily during warm-up.

Utilize postmaster tools to track your new IP's reputation and performance.

Common pitfalls

Sending large volumes too quickly from a new IP, leading to immediate blocklisting.

Ignoring low engagement from less active segments, which can damage reputation.

Failing to monitor deliverability metrics closely, missing early warning signs.

Not signing up for feedback loops, missing crucial spam complaint data.

Sending to uncleaned lists, resulting in high bounce rates on the new IP.

Expert tips

Consider a dual-IP strategy where some traffic temporarily remains on the old IP while warming the new one, if your ESP supports it.

Even if content is time-sensitive, sacrifice initial reach for long-term deliverability by sticking to your warm-up schedule.

Educate your team about the importance of IP warm-up and its impact on email marketing goals.

Ensure your DNS records (SPF, DKIM, DMARC) are correctly configured for the new IPs before beginning your warm-up.

Maintain consistent content quality and relevance throughout the warm-up to encourage positive recipient engagement.

Expert view

Expert from Email Geeks says: Prioritize your most engaged subscribers first during IP warm-up, as this segment typically generates the highest opens and lowest complaints, which is crucial for building new IP reputation.

July 23, 2020 - Email Geeks

Marketer view

Marketer from Email Geeks says: Carefully consider the content relevance for different segments during warm-up, ensuring that emails sent to less engaged groups do not negatively impact your sender reputation by increasing spam complaints.

July 23, 2020 - Email Geeks

Final thoughts on your IP warm-up

An ESP server migration, particularly one that involves new IP addresses, requires a deliberate and well-executed IP warm-up strategy. Your ability to maintain email deliverability and avoid being blocklisted (blacklisted) hinges on how effectively you build a positive reputation for your new sending IPs. By prioritizing your most engaged audiences, gradually increasing your sending volume, and meticulously monitoring your email metrics, you can navigate this transition smoothly.

While the process may seem daunting, especially with large, time-sensitive sends, a strategic approach minimizes risk and safeguards your sender reputation. With careful planning and consistent execution, your email program can continue to thrive on its new infrastructure.