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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[–] schwim@lemm.ee 2 points 4 months ago (1 children)

There's really not enough info here to help you. Are you looking for software? Writing it from scratch? Web tool? Bulk or not?

I don't know how many addresses you plan on testing on any one server but we've been on to this trick for decades now and the firewall will block you from almost every server once you try a non-existent address a few times(for my servers, it's 2). Many servers also report bot/spam IPs to the ISP and if you get reported enough time, your connection could get shut down.

[–] boredsquirrel@slrpnk.net 1 points 4 months ago (1 children)

Damn, thanks for the info.

Definetly not wanting to land on a spam list...

So I will test the mails with burner accounts? Which would be a bit sus, but I could just write "DO YOU WANT TO STRENGTHEN YOUR ERECTION" and that would end in spam anyways

[–] schwim@lemm.ee 4 points 4 months ago* (last edited 4 months ago)

That might not work either. If a server marks it as spam, we do something called blackholing the email, meaning we discard the email and close the connection without responding to the sending server. This is done in an effort to provide as little info as possible to a bad actor.

If you don't send an email from a server and address deemed reputable and with a low enough spam score, you'll be shut down by more than 95% of the mail servers out there.