Email outreach
Email warmup is not enough: the sender reputation checklist for AI outreach
By Viktorio Shopov, co-founder of Sellinger. Reviewed by Asen Levov. Published . Last updated . 8 minute read.
Warmup only controls one thing, how fast your daily volume rises. Whether your mail reaches the inbox depends on authentication (SPF, DKIM, DMARC), the quality of the list, the share of recipients who mark you as spam, how many addresses bounce, and whether people reply. Google tells senders to keep the spam rate reported in Postmaster Tools below 0.10% and never to reach 0.30%. Treat warmup as the ramp, and the other eight items on the checklist below as the road.
Most advice on cold email deliverability starts and ends with "warm up your inbox for two weeks". That is a fair first step for a new mailbox and a poor summary of what mailbox providers look at. When an AI agent writes and sends the mail, the other items matter more, because the agent can send faster and further than a person ever would.
What does warmup actually do?
A warmup is a schedule that raises the number of emails a mailbox sends each day from a low start to a target. The point is to avoid a pattern that looks unusual: a mailbox that sent nothing last week and 200 messages today.
Sellinger's optional warmup follows a fixed rule. It starts at 5 emails a day and doubles every 3 days until it reaches the mailbox's daily limit, which defaults to 50:
| Days of sending | Daily limit during warmup |
|---|---|
| 1 to 3 | 5 |
| 4 to 6 | 10 |
| 7 to 9 | 20 |
| 10 to 12 | 40 |
| 13 onward | 50 (the mailbox limit) |
The limit is a setting per mailbox, and it is off unless you switch it on. A mailbox with a long, healthy sending history may not need it. A brand new one probably does.
That is all a warmup does. It does not authenticate your domain, clean your list, write a message people want, or stop recipients from complaining. Those belong to the other eight items.
The nine point checklist
1. Authentication
Google's sender guidelines require bulk senders, defined as those sending over 5,000 messages a day to Gmail, to set up SPF, DKIM and DMARC for the sending domain. If you send less, the same records are still the minimum that mailbox providers expect. These are records on your own domain, set at your DNS and mailbox provider, and the minimum setup post walks through them. Sellinger sends through your connected mailbox and does not set these records for you.
2. A separate sending domain, if it is warranted
Many teams send cold mail from a domain close to the main one, so a problem does not touch the mail that carries invoices and customer replies. It is a judgement call and we publish no rule. If you choose it, authenticate and warm up the second domain on its own.
3. List quality
A list with wrong addresses produces bounces, and bounces are the quickest way to a poor reputation. Google advises that when messages start bouncing or being deferred you should reduce the sending volume until the error rate decreases. Verify addresses before the send, as covered in email verification before you send.
4. Spam complaints
Google asks senders to keep the spam rate shown in Postmaster Tools below 0.10%, and bulk senders must stay under 0.30%. At 0.10% that is one complaint per 1,000 messages. A message that looks like a mass mailing, offers nothing to the recipient or ignores an earlier "no" is the kind of message that gets reported.
5. A way to leave
Google requires marketing and subscribed messages to support one click unsubscribe, with the List-Unsubscribe and List-Unsubscribe-Post headers, and a visible unsubscribe link. In Sellinger, cold email can carry an unsubscribe line and those headers as a per mailbox setting, which is off by default, so switch it on for the mailboxes where it applies. Whatever route you choose, stop when someone asks. Sellinger checks the team's suppression list before every send.
6. Volume and pacing
Daily caps matter more with an AI agent, since it will use all the budget you give it. Choose the limit on purpose: start below the maximum, raise it in steps, and watch the signals in items 3 and 4 before each step.
7. Message quality
Short, specific, human. A message of three to five sentences with one observation about the recipient and one easy question looks very different to a recipient, and to a filter, from a long template. The length post and the rules we enforce cover this.
8. Replies
Replies are the signal a mailbox wants: a real person is answering a real person. In the Hotel Pirin case study, the agent wrote cold email from the hotel's own sales mailbox to 5,499 B2B contacts between 22 July and 11 October 2026, and 163 replied (3.0%), counting refusals and excluding auto replies. Sellinger also recognises helpdesk auto replies, no reply senders and bounce notices as machine mail, so they are not counted as a human reply and the agent does not answer them.
9. Monitoring and a stop rule
Check Google Postmaster Tools for the domain at least weekly, and decide in advance what makes you stop. For example: stop sending from a mailbox for 48 hours if the bounce rate on a day's sends exceeds a threshold you chose, and reduce volume when the spam rate approaches 0.10%. A written rule is worth more than a promise to watch.
How do these fit together for an AI agent?
An agent sends at the speed of its settings. That cuts both ways. It makes pacing exact, since the limit is enforced in code, and it makes a mistake expensive, since a bad list can be sent to in an afternoon. So the order of work is:
- Authenticate the domain.
- Verify the list.
- Switch on the unsubscribe line and headers.
- Start in approval mode so a person reads the first batch.
- Start the warmup, or set a low daily limit.
- Read the Postmaster Tools numbers after the first week, then raise the limit one step.
What can warmup not fix?
- A list of unverified addresses.
- A domain with no SPF, DKIM or DMARC.
- A message that recipients report as spam.
- A sender that ignores an unsubscribe or an opt out.
- A mailbox shared with another business that has a bad reputation.
If your problem is on this list, adding more warmup days only delays the point at which it shows.
A checklist
- [ ] SPF, DKIM and DMARC are set on the sending domain.
- [ ] I decided on purpose whether to use a separate sending domain.
- [ ] Every address is verified before it is sent to.
- [ ] The unsubscribe line and one click headers are on where they apply.
- [ ] The daily limit and the warmup are set, below the maximum.
- [ ] The first batch went through approval.
- [ ] Postmaster Tools is checked weekly.
- [ ] I have a written stop rule for bounces and complaints.
Common questions
Is email warmup worth doing?
Do warmup tools that exchange emails between inboxes work?
What is the most common reason cold email lands in spam?
How many emails a day is safe?
What should I do if bounces or spam complaints rise?
How Sellinger helps
Read how it works on See the email agent, or see a real campaign in Read the Hotel Pirin case study.
