Quote:
Originally Posted by ao1
... How do I put a whole domain in my address book?
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Add a contact with the email address of *@domain.tld. For example:
*@example.org
That will whitelist the domain for the spam filter, but in your example the message was rejected at the SMTP acceptance stage. Based on a post I saw somewhere else, I think that Fastmail has used such a new domain block for at least a couple of years or so to reduce spam from new domains. Spammers often create a new domain so they have a while to send their junk before it's blocked. I'm not sure exactly how the server makes a decision that the domain is "too new", and I'm not sure if address book whitelisting will help in this case.
Your example had a "451" SMTP error. This is the usual response when a message is deferred by greylisting. Most SMTP outgoing servers will re-send the message later. The sending server should not give up at the first 451 error but should try again later. In this case (new domain) I'm not sure how Fastmail applies greylisting, and they probably don't want to publicize their exact policy to prevent creative spammers from evading their greylist techniques.
See the
Greylisting section in this help page:
https://www.fastmail.com/help/technical/smtpchecks.html
Bill