What is IP warming and why is it necessary?

IP warming (or ramp-up) is the process of gradually increasing the volume of emails sent from a new IP address. ISPs track the reputation of sending IPs, and a new IP has no history. A sudden burst of email from a cold IP looks like spam, so a gradual increase helps build a positive reputation and trust with mailbox providers, leading to better inbox placement.

Does my existing domain reputation transfer to a new IP address?

When you migrate ESPs, your domain's reputation largely carries over, but your new IP address starts with a neutral reputation. ISPs evaluate both. If your new IP sends too much too soon, or has authentication issues, it can negatively impact your overall deliverability, even if your domain reputation is strong. The new IP needs to build its own sending history.

What technical issues commonly cause emails to go to spam after an ESP migration?

Common technical issues include incorrect SPF, DKIM, or DMARC records that fail to authenticate your emails from the new ESP. This tells ISPs that your emails might be spoofed or unauthorized. Other issues can include missing reverse DNS (rDNS) or inconsistent sending domains/subdomains.

How do recipient engagement metrics affect deliverability during IP warming?

A spike in spam complaints, high bounce rates, or low open/click rates indicate problems. ISPs view these as signs of unwanted mail. If recipients mark your emails as spam, or if a large portion of your list is inactive, it will negatively impact your sender reputation, especially on a new IP.

What causes emails to go to spam when migrating to a new ESP and warming an IP? - Sender reputation - Email deliverability - Knowledge base

Migrating to a new Email Service Provider (ESP) is a significant undertaking that promises enhanced features and improved sending infrastructure. However, it often comes with an unexpected side effect: a sudden drop in email deliverability, with many emails landing in the spam folder. This issue is particularly prevalent when a new IP address is introduced, requiring a careful IP warming process.

The core of the problem lies in sender reputation. Internet Service Providers (ISPs) and mailbox providers, such as

Gmail and

Outlook, heavily rely on historical sending data to determine whether an email is legitimate or spam. When you switch ESPs and begin sending from a new IP address, that historical data disappears. It's like starting from scratch, and without a proven track record, ISPs are inherently cautious.

During this period, even legitimate emails sent to engaged subscribers can be flagged. ISPs often employ aggressive filters on new or cold IP addresses because spammers frequently use fresh IPs to evade detection. Understanding why your emails might be hitting the spam folder is the first step toward successful deliverability.

This guide explores the primary reasons emails go to spam during an ESP migration and IP warming, and provides actionable insights to help you navigate this critical transition.

The reputation reset challenge

When you migrate to a new ESP, you typically acquire a new set of IP addresses for sending. This move essentially resets your sending reputation in the eyes of ISPs. Unlike your previous ESP, which had years of established sending history, your new IP address has no prior record. Mailbox providers interpret a sudden surge of mail from an unknown IP as suspicious, often flagging it as potential spam until it proves otherwise.

IP warming (or ramp-up) is the controlled process of gradually increasing your email volume from a new IP address. This slow and steady approach allows ISPs to observe your sending behavior, build a positive sending history, and ultimately trust your mail. Without proper warming, an abrupt high volume of email from a cold IP can trigger spam filters, leading to immediate blocklisting or heavy inboxing to the spam folder.

A common mistake is assuming that if your domain reputation is strong, your new IP will automatically inherit that trust. While domain reputation plays a crucial role, a new IP still needs to establish its own credibility. ISPs look at both IP and domain reputation. If your new IP is sending an uncharacteristically high volume, it can override the positive signals from your domain, resulting in emails landing in spam.

The Challenge

No history: Your new sending IP has a neutral, or even negative, reputation initially because it lacks a track record of sending good mail.
ISP suspicion: Mailbox providers are wary of new IPs due to historical abuse by spammers, so they apply stricter filtering.
Volume spikes: Sending too much too soon from a cold IP is a red flag and can lead to immediate blocklisting (or blacklisting).

Technical configuration pitfalls

Beyond reputation, technical misconfigurations are a frequent culprit for emails landing in spam. When you switch ESPs, you often need to update your DNS records, including SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting & Conformance). These records are vital for authenticating your emails and proving to ISPs that you are a legitimate sender.

An improperly configured SPF record might not authorize your new ESP's sending IP addresses, leading to authentication failures. Similarly, an incorrect DKIM setup can break your email's digital signature, making it appear tampered with. DMARC, which builds upon SPF and DKIM, instructs receiving servers how to handle emails that fail authentication. If your DMARC policy is set to 'quarantine' or 'reject' (p=quarantine or p=reject) and your authentication fails, your emails will go directly to spam or be blocked.

SPF record example for a new ESPDNS

v=spf1 include:_spf.newesp.com ~all

Sometimes, even if DNS records are correct, other technical elements can cause problems. Inconsistent From headers or reply-to addresses, a lack of reverse DNS (rDNS) for your sending IP, or even using a generic subdomain for sending can raise red flags with spam filters. It's crucial to ensure all technical aspects of your email setup are meticulously aligned with your new ESP's requirements and industry best practices.

Content and audience engagement impact

Mailbox providers closely monitor how recipients interact with your emails. When you migrate, your sending behavior comes under increased scrutiny. Engagement metrics like open rates, click-through rates, and, crucially, spam complaint rates, heavily influence your new IP's reputation. A sudden drop in engagement or a spike in complaints, even if from a small portion of your list, can severely damage your standing.

Sending to unengaged or outdated email lists is a major pitfall during warming. ISPs track recipient engagement signals very closely. If a large percentage of recipients aren't opening or clicking your emails, it signals to ISPs that your mail is unwanted. Worse, sending to inactive addresses can hit spam traps (or honeypots), which are designed to catch spammers and can lead to immediate blocklisting.

The content of your emails also matters. Certain keywords, excessive capitalization, too many images, or a lack of plain text alternatives can trigger spam filters, especially on a new IP. While these factors might have been tolerated on an established IP, they become much riskier during the warming phase when trust is still being built. The goal is to send only the most wanted, relevant content to your most engaged subscribers initially.

Bad practices during warm-up

Sending to entire list: Starting with your full list, including unengaged subscribers.
Ignoring engagement: Not monitoring open rates, clicks, and complaints closely.
Spammy content: Using excessive promotional language or poor formatting.

Good practices during warm-up

Segmenting audience: Sending to your most active and engaged subscribers first.
Active monitoring: Adjusting sending volume based on real-time feedback and metrics.
High-value content: Focusing on transactional or highly anticipated content initially.

Monitoring and adaptation during warm-up

During and after your IP warming period, vigilant monitoring of your email performance is non-negotiable. Deliverability issues can manifest in various ways, from direct bounces to silent filtering into spam folders. Utilizing tools like Google Postmaster Tools and other postmaster feedback loops is essential for gaining insights into your sending reputation and identifying potential problems early.

You should regularly check if your IP address or domain has landed on any email blocklists (or blacklists). Being listed on a major blocklist means that many ISPs will outright reject or spam-folder your emails, regardless of your content or audience. Prompt action to request de-listing is crucial if this occurs.

Adaptability is key. If you notice a dip in deliverability or an increase in complaints, be prepared to adjust your sending strategy. This might mean slowing down your sending volume, further segmenting your audience to target only the most engaged, or refining your email content. Remember, the goal is to build a positive and consistent sending reputation over time, which requires patience and responsiveness to feedback.

Metric	Why it matters	Action if poor
Inbox placement rate	Directly indicates how many emails reach the inbox versus spam.	Reduce volume, check content for spam triggers.
Spam complaint rate	High complaints severely damage sender reputation.	Remove unengaged subscribers, review content and list hygiene.
Open rate	Indicates recipient interest and validates content relevance.	Improve subject lines, send to more engaged segments.
Bounce rate	High bounce rates indicate a poor quality list.	Clean your list, remove invalid addresses.

For diagnosing and resolving deliverability drops, a systematic approach is crucial. This involves not only technical checks but also a deep dive into your audience segmentation and content strategy.

Views from the trenches

Best practices

Thoroughly clean your email list before migration to remove inactive or questionable addresses.

Segment your audience and start sending to your most engaged subscribers first during warming.

Consistently monitor key metrics like open rates, click-through rates, and spam complaints.

Ensure all DNS records (SPF, DKIM, DMARC) are correctly configured for your new ESP.

Be prepared to adjust sending volumes and content based on real-time deliverability feedback.

Common pitfalls

Sending the same volume of emails from a new IP as you did from your old, established one.

Failing to update all necessary DNS authentication records correctly after the migration.

Not monitoring email deliverability metrics closely, leading to prolonged spam placement.

Ignoring feedback from postmaster tools and DMARC reports.

Sending to an unsegmented list that includes old, inactive, or unengaged subscribers.

Expert tips

ISPs test user interactions by delivering emails in a mix of inbox and spam during warm-up.

New IP addresses require a higher standard of engagement, data quality, and relevance.

Warm up your IP with the type of emails you intend to send regularly, not old patterns.

Confirm that email addresses belong to active customers, not just that they won't bounce.

Expect a temporary drop in deliverability and be ready to adapt your sending strategy.

Marketer view

Marketer from Email Geeks says that filters deliver in a mix of inbox and spam, watching for people to move messages, as they are testing recipient interactions.

2018-12-11 - Email Geeks

Marketer view

Marketer from Email Geeks says that if you have a way to contact users outside of email, telling them to expect mail and check their spam folder helps.

2018-12-11 - Email Geeks

Navigating your migration successfully

Migrating to a new ESP and warming an IP address can feel like navigating a minefield, but understanding the underlying causes of deliverability issues is the first step toward success. The primary reasons emails go to spam during this period boil down to a lack of established sender reputation, potential technical misconfigurations, and suboptimal sending practices that fail to meet ISPs' heightened scrutiny.

By meticulously planning your IP warming schedule, ensuring all authentication records (SPF, DKIM, DMARC) are correctly set up, and focusing on sending high-quality, engaged content to your most active subscribers, you can minimize the risk of spam placement. Continuous monitoring and a willingness to adapt your strategy based on real-time feedback are also critical. Patience and persistence will ultimately pay off, allowing you to establish a strong, positive sending reputation with your new ESP.