Glossary
List hygiene
Also called list cleaning, sunsetting.
Last reviewed 18 September 2026
What is list hygiene?
List hygiene is the work of keeping a list to addresses that exist and belong to people who still want the mail: hard bounces suppressed on the first failure, long-silent addresses retired, typos caught at the form, and nothing re-imported that was already suppressed.
The sunset window is the part that needs a number, and the honest answer is a range: most senders stop after somewhere between three and six months with no measurable engagement. A course sender can be stricter than a newsletter, because a course either gets finished or it does not.
What matters more than the exact window is that there is one and that it applies automatically. Hygiene done by hand is hygiene done when somebody remembers, which is after the complaint rate has already moved.
See also
- Hard bounce
A hard bounce is a permanent refusal of a message by the receiving server, carried in a 5xx SMTP response, because the mailbox does not exist, the domain does not exist, or the address has been disabled.
- Suppression list
A suppression list is the permanent record of addresses that must never be mailed again, holding unsubscribes, complaints and hard bounces, and it is checked before every send rather than after one.
- 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.
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.