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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[–] Max_P@lemmy.max-p.me 9 points 3 months ago (1 children)

On top of everything the others have said, another way this isn't possible reliably is servers that just accept all email and forward it to a catchall address.

Some also have trap addresses where sending email to it will result in putting that address directly into the spam filter and everything coming from it feeds into training the spam filter. I'm an individual, not a company, so all the common IT, HR, support, press, sales, whatever addresses are traps.

When websites force me to enter an email, I enter one of the traps so everything they send me and everyone they share that email with gets the banhammer instantly, and I can track which asshole website did that to me as well.

[–] 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.

[–] schwim@lemm.ee 2 points 3 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 3 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 3 months ago* (last edited 3 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.

[–] Shadow@lemmy.ca 2 points 3 months ago

This hasn't been possible for a long time. Mail servers do not typically reject a bad recipient immediately on the SMTP connection, they accept it and send a bounce email afterwards instead.

[–] lemmyreader@lemmy.ml 2 points 3 months ago

I think to know that you can query some info from a mailserver to test if an address exists.

Yes, vrfy

For Postfix the default of disable_vrfy_command is : no So I assume you can try it risk free.

[–] h3ndrik@feddit.de 1 points 3 months ago* (last edited 3 months ago)

Not possible. Almost all mailservers have migitated for this kind of thing. Even if you wrote a script, it wouldn't work on any properly configured mailserver..

[–] muntedcrocodile@lemm.ee 1 points 3 months ago (1 children)

I mean there is always the option to just send emails with an image link that auto loads and detect if someone loads it.

[–] boredsquirrel@slrpnk.net 2 points 3 months ago (1 children)

Interesting idea! But if servers block me after sending x mails to nonexistent inboxes, this doesnt help.

[–] muntedcrocodile@lemm.ee 3 points 3 months ago

Well now its simply a matter of how many email servers can u get emailing on your behalf. U can buy a list of 300 google accounts for about $3 (general email:pass lists are far cheaper but lower success rate) add a proxy network and a python script and u can bot em all. Use ai and url shortener to make ur email content harder to filter. Obviously this is hypothetical and i do not condone such activities.