Email warm-up for new IPs and domains

Warm-up means raising the volume from a new sending IP address or domain gradually, so mailbox providers can learn that its mail is wanted before it sends at full volume. A new IP or domain has no history; a sudden flood from it looks like a spammer's. Start with your most engaged recipients, increase steadily, and slow down at the first sign of deferrals, bounces or complaints. No large provider publishes a fixed schedule.

Why new IPs and domains need it

Mailbox providers judge mail partly by the sender's past behavior. Microsoft says that IP addresses that have never sent mail typically have no reputation in its systems, so their mail is more likely to run into delivery problems until they build one (Troubleshooting mail sent to Microsoft 365). Google's email sender guidelines warn against sudden volume spikes from a sender without a history of large volumes, and ask senders to watch both their domain's and their IP's reputation in Postmaster Tools.

Two kinds of warm-up are worth separating:

• IP warm-up: you move to a dedicated IP address, or add one. The IP has no history even if your domain does.
• Domain warm-up: you start sending from a new domain or subdomain. Even on an email service provider's shared, already warm IPs, the new domain's own reputation starts from nothing.

Moving to a new email service provider usually means both, unless you keep your domain and the provider puts you on its shared IPs.

What the providers say

Google gives the most detail, in the "increase sending volume slowly" part of its sender guidelines:

• Larger volumes call for a slower ramp.
• Start with your most engaged users and add others over time. Daily sending lets you increase faster than weekly sending.
• Send at a steady rate rather than in bursts, and avoid sudden spikes.
• When you change a message format or your sending infrastructure, ramp up the affected traffic gradually and on its own.
• Watch SMTP responses, spam rates and reputation in Postmaster Tools. If messages start to bounce or are deferred, lower the volume until the errors drop, then increase slowly again.
• If Gmail answers with 421 4.7.28, you have exceeded a rate limit: wait at least 10 minutes without sending, then resume with a single connection and add connections one at a time.

Microsoft estimates that a new IP reaches full volume in about two weeks at most, and sooner the smaller your volume, the more accurate your list and the fewer junk-mail complaints you get. New IPs on a domain with an existing SPF record and a good reputation inherit some of that reputation and may ramp faster (Outlook.com Postmaster troubleshooting). If you are enrolled in its Junk Email Reporting Program, add the new IPs there too.

Yahoo publishes no warm-up schedule in its sender best practices. It does ask you to keep marketing mail on different IPs from transactional and alert mail, because each IP and each DKIM domain has its own reputation, and it closes a connection without an error once a per-connection message limit is reached; you can then reconnect.

Before you send the first message

Warm-up only works if the mail would pass anyway. Check these first:

1. Authentication. SPF, DKIM and DMARC for the new domain or IP, with the From: domain aligned. Gmail and Yahoo require this of bulk senders, and Microsoft of high-volume senders (Gmail and Yahoo, Outlook). Test with the SPF, DKIM and DMARC checkers. Google's FAQ also puts new domains on a faster enforcement timetable, so fix this before volume, not after.
2. Reverse DNS. The IP needs a PTR record that resolves to a host name pointing back to the same IP; a generic, dynamic-looking name works against you (reverse DNS for mail servers).
3. A clean list. Bounces during warm-up hurt more than at any other time, because the new IP or domain has nothing to offset them. Verify the addresses you plan to use with bulk email verification and leave out everything with the action remove; leave catch_all and unknown results for after the warm-up (list cleaning).
4. Engagement data. Pick the recipients who opened, clicked or bought recently. They are your first days.
5. Monitoring. Register the domain in Google's Postmaster Tools and the IPs in Microsoft's SNDS, and make sure you can see bounce and deferral codes per receiving domain in your sending platform.

An example ramp

The schedule below is an example only, not a rule from any provider. It assumes a sender who will eventually send about 50,000 messages a day, mails daily, and starts with recipients who engaged in the last 30 days.

Adapt it to your numbers: a smaller sender reaches full volume sooner, a larger one needs more steps. The point is the rule behind it, which follows Google's guidance: move to the next step only when the last one was clean, and step back when deferrals or bounces appear.

Signals to watch

• Deferrals (4xx codes): 421 4.7.28 is Gmail's rate limit, and 421 4.7.0 a temporary refusal, often for reputation. Hold the volume, or step back, until they stop.
• Hard bounces (5xx codes). Remove those addresses at once. A rising rate during warm-up usually means a list problem, not a volume problem (bounce codes).
• Spam complaints. Gmail and Yahoo ask for a spam rate below 0.3%, and Google recommends below 0.1%. A rise means you reached people who do not expect your mail.
• Inbox placement and engagement of the first segments compared with your old setup.
• Blocklist listings of the new IP or domain (email blocklists).

When something goes wrong, the fix is the same at every provider: lower the volume, return to your most engaged recipients, and find the cause before you increase again.

Questions

How long does warm-up take?

There is no fixed answer. Microsoft speaks of a couple of weeks or less for a new IP, depending on volume, list accuracy and complaints; Google gives no duration and ties the pace to your volume and the responses you get. Larger volumes take longer.

Do I need to warm up if my provider uses shared IPs?

Usually not the IP: shared IPs already send mail every day. Google points out that everyone on a shared IP affects its reputation (shared vs dedicated IPs). A new domain or subdomain still has no history, so raise its volume gradually too.

Are automated warm-up services a good idea?

None of the three providers' guidelines describe them. Their advice is about real recipients who want your mail: engaged users first, steady volume, low complaints. Build your reputation on that.

Should I warm up transactional and marketing mail together?

Keep them apart where you can. Yahoo asks senders to send marketing mail from different IPs than transactional mail, and Google suggests consistent From: addresses per type of message and a separate IP per type if you use several (separating transactional and marketing mail).

Try it

Verify your warm-up segments with bulk email verification, check your DNS records with the DNS tools, and look up any deferral or bounce in the bounce code dictionary.

Related

• Glossary: sender reputation and deliverability.
• Reduce your email bounce rate: the per-campaign checklist.

Related guides

• Gmail and Yahoo sender requirements
• Outlook.com requirements for high-volume senders
• Reduce your email bounce rate
• How to clean an email list
• Double opt-in (confirmed opt-in)
• All guides

Sources

• Google: Email sender guidelines: increasing sending volume, rate limits, shared IPs.
• Google: Email sender guidelines FAQ: enforcement for new domains.
• Outlook.com Postmaster: Troubleshooting: new IP addresses.
• Microsoft Learn: Troubleshooting mail sent to Microsoft 365.
• Yahoo: Sender best practices: separate mail streams, flow control.

How we wrote this: drafted by the email-validation.org team with the help of AI writing tools, from our product documentation and the sources linked in the text.