Single opt-in
A signup in which an address is added to the list as soon as it is submitted, without confirming that its owner made the request.
What it means
M3AAWG describes single opt-in as users providing an address and starting to receive mail, with no notification and no confirmation of consent. It says single opt-in should be used with extreme caution because it lets unconfirmed addresses onto lists: typos and forged submissions go through unchecked, creating invalid consent and leading to complaints, bounces and a gradual loss of reputation.
A middle level, single opt-in with notification, sends a welcome message at once (within 24 hours, M3AAWG suggests) stating what was submitted and what will follow, and removes the address if that message bounces.
Single opt-in lists are where Spamhaus recommends a permission pass if they start causing problems.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes