Mailserver block the requests that are used to get a list of inboxes ("accounts")
many servers will block mailservers that are not on an allowlist
many servers will block servers, if mails were sent to nonexistent addresses a couple of times
the message "recipient not known" will not appear often, as servers may "black hole" a senders mail and cut off the connection without sending the status message back

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[–] boredsquirrel@slrpnk.net 1 points 3 months ago

Very smart. Learned something.

Many many company mains are standard, which is interesting as they will get tons of spam.

But makes sense that this is not a good way, that the tools for scanning for available addresses are blocked and that there are even honeypots.