Deliverability to the inbox, rather than spam, depends not only on correct SPF, DKIM and DMARC records, but also on the reputation of the sending IP address and domain. A new server or a new domain has no reputation yet: mail systems such as Gmail, Outlook and Mail.ru build trust in a sender gradually, over several weeks, before handing the full volume of mail to it at once. This article covers how sender reputation is built, how to warm it up, and which signals to watch if mail still ends up in spam.
What sender reputation is made of
Reputation is not a one-off mark inside a single message but an accumulated history of an IP address and a domain: how many messages came from them, what share was rejected or marked as spam, how many recipients complained, and how stable the sending volume was over time. Mail systems store this history separately for the IP and for the domain, so changing the IP does not reset a domain's bad reputation, and the other way round.
Reputation is also shaped by the surrounding IP space: if an IP address previously belonged to a spammer, or sits in a range where neighboring addresses send bulk unwanted mail, part of the trust is lost before the first message leaves that server. A dedicated static IP, not tied to the history of a shared-purpose provider, therefore usually starts from a better position than a random address from a cloud pool.
Warming up a new IP and domain
Warm-up is a gradual increase in sending volume from a new IP or domain so that mail systems have time to build up statistics without sharp jumps. A typical approach is to start with a small batch sent to the most responsive recipients, who reliably open mail, and raise the volume to the working level over 2-4 weeks while watching the bounce and complaint rates.
A sudden start with a large volume is one of the most common reasons a new server gets blocked: the mail system sees a jump from zero messages to thousands per day and reacts as if to likely spam, even if the content is fully legitimate. It is better to stretch the warm-up over several weeks than to shorten it and end up blocked, after which the reputation takes months to rebuild.
Signals that damage reputation
Two metrics usually matter more than the rest: the hard bounce rate and the spam complaint rate. A hard bounce rate above 2% of a batch signals a dirty address list — nonexistent mailboxes, typos, old addresses nobody removed; a complaint rate above 0.1% is already read by large mail systems as a sign of unwanted mail.
The third signal is an unstable volume: if a server was silent for a month and then sent thousands of messages in a day, it looks like a compromise or a change of purpose, and the mail system temporarily lowers trust until it sees the new volume holding steady.
How to check current reputation
Google Postmaster Tools and Microsoft SNDS show IP and domain reputation directly — the share of mail recipients at Gmail and Outlook mark as spam, delivery trends, and listing on their internal lists. Access is free but requires proving domain ownership through a DNS record, similar to the verification used when setting up SPF, DKIM and DMARC.
Public blocklists (DNSBL) are a separate source: the same IP can look clean to Google while sitting on the Spamhaus or SORBS list because of an old complaint. Checking several lists with dig takes seconds and should be the first step whenever bounce rates rise without an obvious reason.
Delivery on your own server and reputation
On a VPS with your own Postfix, warm-up and reputation are entirely your responsibility, unlike large mail services where the provider has already warmed up a shared IP pool for all customers. If mail is set up through Postfix on your own server, warm-up needs to be planned in advance, before moving production mail over, not after the first block.
Understanding the basic path a message takes from sender to recipient mailbox helps pinpoint faster where delivery is lost: a detailed walkthrough is in the article "How email works".
- The batch starts right away at full list volume, with no warm-up
- The address list has not been cleaned of bounces and complaints for over a year
- The subject and body stay identical from one batch to the next
- There is no unsubscribe mechanism, or it is hidden deep inside the message
- The server's IP address previously belonged to another tenant with an unknown history
| Metric | Safe level | Cause for concern |
|---|---|---|
| Hard bounce rate | up to 2% | above 5% |
| Spam complaint rate | up to 0.1% | above 0.3% |
| Daily sending volume (on a new IP) | 50-500 | jump with no warm-up |
| Warm-up period for a new domain | 2-4 weeks | less than 1 week |
# Checking an IP against the main blocklists
# The server IP is written with its octets in reverse order
dig +short 4.3.2.1.zen.spamhaus.org
dig +short 4.3.2.1.bl.spamcop.net
# A non-empty reply means the IP is listedFrequently asked questions
How long does warming up a new IP for mailing take?
Usually 2-4 weeks with a gradual volume increase, but the exact time depends on list size and how responsive the first recipients are: the higher their open rate and the lower their complaint rate, the faster mail systems build up trust.
Can warm-up be sped up by raising volume faster?
Speeding up warm-up raises the risk of a sharp rise in bounces and complaints, which the mail system reads as a sign of spam rather than growth of a legitimate batch. It is safer to keep the planned 2-4 week schedule than to shorten it and end up blocked, after which rebuilding reputation takes longer than the warm-up itself.
What to do if mail from an IP with a good reputation still ends up in spam?
In that case the cause is usually not the IP but the message content, missing or incorrect DKIM and DMARC records, or a mismatch between the domain in the From header and the domain that passed the SPF check. It is worth checking the records through Google Postmaster Tools and re-verifying the DKIM and DMARC setup.