Glossary
Sending domain
Last reviewed 18 September 2026
Definition
A sending domain is the domain that signs and sends your mail, and it is worth separating from the sending infrastructure: the IP addresses mail leaves from are shared by default at every provider serving small senders, while the domain in the signature and the From line is yours alone.
The decision that matters for a small sender is therefore not shared against dedicated IP addresses. IP reputation is built by sustained volume, and a dedicated IP under a low-volume sender is an IP with no reputation at all, which is worse than a share of a well-run pool.
What there is to decide is which of your domains does the sending. Use a subdomain reserved for it and keep the root domain for the mail humans send each other, because a burned subdomain can be replaced in an afternoon and a burned root domain cannot be replaced at all.
See also
- Sender reputation
Sender reputation is what a mailbox provider has learned about the domain and the infrastructure your mail comes from, assembled from complaints, bounces, spam trap hits and how the people who receive your mail behave, and it is what decides the folder.
- Domain warming
Domain warming is the practice of raising the volume sent from a new domain gradually, over something like two to four weeks, so that a domain with no reputation does not arrive looking like one registered this morning for a campaign.
- DKIM
DKIM is a cryptographic signature over a message’s headers and body, made with a private key held by whoever sends the mail and checked by the receiver against a public key published in DNS under a selector, and it is defined in RFC 6376.
- Alignment
Alignment is the DMARC requirement that the domain a reader sees in the From header match a domain that passed SPF or DKIM, which is why a message can pass both of those checks and still fail DMARC.
Read next
This term is covered properly in Email deliverability for small senders: authentication and reputation, which is a longer piece on the subject.
Every other term is on the glossary index , the longer writing is in the guides, and what this site itself does is on the home page.
Thinking of writing one of these?
5dayemail hosts a five-to-ten day email course: you write it once, and everyone who joins your list gets one email a day, in order, starting from the day they confirm.
Accounts are opened a few at a time rather than by signing up. Leave your address and you will be written to when the next ones open.
One message, when there is room. No course emails, no newsletter, and the address is not passed on. Ask and it is deleted; what is kept, and for how long, is in the privacy policy.