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I absolutely love the email protocols.

IMAP:
x1 LOGIN user@domain password
x2 LIST "" "*"
x3 SELECT Inbox
x4 LOGOUT

Because a state machine is clearly too hard to implement in server software, clients must instead do the state machine thing and therefore it must be in the IMAP protocol.

SMTP:
I should be careful with this one since there's already more than enough spam on the interwebs, and it's a good thing that the "developers" of these email bombers don't know jack shit about the protocol. But suffice it to say that much like on a real letter, you have an envelope and a letter inside. You know these envelopes with a transparent window so you can print the address information on the letter? Or the "regular" envelopes where you write it on the envelope itself?
Yeah not with SMTP. Both your envelope and your letter have them, and they can be different. That's why you can have an email in your inbox that seemingly came from yourself. The mail server only checks for the envelope headers, and as long as everything checks out domain-wise and such, it will be accepted. Then the mail client checks the headers in the letter itself, the data field as far as the mail server is concerned (and it doesn't look at it). Can be something else, can be nothing at all. Emails can even be sent in the future or the past.

Postfix' main.cf:
You have this property "mynetworks" in /etc/postfix/main.cf where you'd imagine you put your own networks in, right? I dunno, to let Postfix discover what your networks are.. like it says on the tin? Haha, nope. This is a property that defines which networks are allowed no authentication at all to the mail server, and that is exactly what makes an open relay an open relay. If any one of the addresses in your networks (such as a gateway, every network has one) is also where your SMTP traffic flows into the mail server from, congrats the whole internet can now send through your mail server without authentication. And all because it was part of "your networks".

Yeah when it comes to naming things, the protocol designers sure have room for improvement... And fuck email.

Oh, bonus one - STARTTLS:
So SMTP has this thing called STARTTLS where you can.. unlike mynetworks, actually starts a TLS connection like it says on the tin. The problem is that almost every mail server uses self-signed certificates so they're basically meaningless. You don't have a chain of trust. Also not everyone supports it *cough* government *cough*, so if you want to send email to those servers, your TLS policy must be opportunistic, not enforced. And as an icing on the cake, if anything is wrong with the TLS connection (such as an MITM attack), the protocol will actively downgrade to plain. I dunno.. isn't that exactly what the MITM attacker wants? Yeah, great design right there. Are the designers of the email protocols fucking retarded?