Glossary
Suppression list
Also called do-not-mail list, exclusion list.
Last reviewed 18 September 2026
What is a 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.
It is the most valuable list you own and the one most often lost. It has to survive every re-import, every change of sending platform and every export to a spreadsheet, because the way somebody gets mailed after they left is almost always a list that went around a suppression check rather than through it.
When somebody exercises a right to erasure you still need to not mail them again, which is the apparent contradiction at the centre of this. It is usually resolved by keeping the minimum necessary to honour the request rather than the record itself: a one-way hash of the normalised address, with no name, no history and no profile attached, so what is retained is a prohibition rather than a subscriber.
See also
- Unsubscribe
An unsubscribe is a reader withdrawing permission to be mailed, and since February 2024 Google and Yahoo have required one to be honoured within two days, a much shorter clock than the 10 business days the United States CAN-SPAM Act allows.
- 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.
- Feedback loop
A feedback loop is an arrangement in which a mailbox provider reports back to a sender the messages its users marked as spam, so that those addresses can be suppressed immediately rather than mailed again.
- 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.
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?
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