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What causes emails to go to spam when migrating to a new ESP and warming an IP?

Summary

Migrating to a new Email Service Provider (ESP) and warming up a new IP address are critical phases for any email sender. During this transition, it is common for emails to experience deliverability issues, including landing in the spam folder, even when sending to an engaged audience. The primary reason for this lies in the reset of sender reputation when a new IP address is introduced. Internet Service Providers (ISPs) and mailbox providers rely heavily on the sending history and engagement metrics associated with a specific IP and domain combination to determine email placement.

What email marketers say

Email marketers often find the transition to a new ESP and the subsequent IP warming process challenging. They frequently report unexpected dips in deliverability, even when following established best practices like sending to engaged audiences. The core frustration stems from the perceived inconsistency, as strategies that worked well with a seasoned IP on a previous ESP don't yield the same results immediately on a new, unproven IP address. This highlights the dynamic nature of sender reputation and the need for adaptability during migration.

Marketer view

A marketer from Email Geeks observed that filters often deliver messages in a mix of inbox and spam during warming, then monitor user interactions to see if messages are moved between folders. This indicates that ISPs are actively testing user responses to new sending patterns. The initial placement is a signal for the system to learn from subscriber behavior.

11 Dec 2018 - Email Geeks

Marketer view

A marketer from Email Geeks asked if push campaigns have been used to prompt people to check their spam folders. This query suggests a proactive approach marketers consider when emails unexpectedly land in spam during a new ESP migration, recognizing that direct communication outside of email might be necessary to guide subscribers. It highlights the challenge of ensuring message visibility during transitional phases.

11 Dec 2018 - Email Geeks

What the experts say

Experts in email deliverability consistently emphasize that moving to a new ESP or a new IP address fundamentally changes how mailbox providers evaluate your email streams. Unlike a long-standing IP that has accumulated years of positive sending history, a new IP lacks this crucial reputation. This absence of history means that ISPs apply stricter scrutiny, requiring senders to adopt even higher standards for engagement, data quality, and relevance to successfully reach the inbox. The past performance on an old IP does not automatically transfer, necessitating a renewed focus on best practices.

Expert view

An expert from Email Geeks stated that you cannot do the exact same things you did on an ESP where you've been mailing for many years on a new ESP. This emphasizes that established reputation allows for more leniency, while a new IP requires a more stringent approach to email practices. Past behavior doesn't guarantee future success with a new sender identity.

11 Dec 2018 - Email Geeks

Expert view

An expert from Email Geeks explained that if you move ESPs, you have to change what you're doing, even after a correct warmup, to meet a higher standard of engagement and relevance. This highlights the evolving nature of deliverability requirements and the need for senders to consistently improve their practices, particularly when establishing a new sending reputation. It is not just about the technical warmup, but the entire email program.

11 Dec 2018 - Email Geeks

What the documentation says

Technical documentation from leading email service providers and industry bodies consistently outlines the necessity and methodology of IP and domain warming. They clarify that warming is a process designed to build sender reputation with ISPs by gradually increasing email volume over a period, allowing filters to assess the legitimacy and quality of the new sending source. The consensus is that skipping or rushing this phase leads to immediate deliverability penalties, as automated systems flag unproven high-volume senders as potential threats. Proper authentication is also a non-negotiable component.

Technical article

Documentation from Amazon Web Services (AWS) explains that IP and domain warming involves slowly increasing sending volume to build a good sender reputation. This process allows mailbox providers to learn legitimate sending patterns. It is crucial for ensuring smooth deliverability and avoiding reputation penalties associated with sudden volume spikes from new, untrusted IPs. The guide emphasizes the importance of planning the transition carefully.

03 Jul 2025 - AWS

Technical article

Documentation from EmailLabs cautions that Microsoft's spam filters are particularly sensitive to sudden increases in email volume from new IPs, potentially leading to blockages. This highlights the specific vigilance of major mailbox providers and the need for a carefully paced warming strategy. Ignoring these sensitivities can result in immediate and severe deliverability problems, underscoring the importance of gradual volume escalation.

12 Dec 2024 - EmailLabs

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