So I got a mail server stood up running Postfix running it's most simplistic configuration for a single domain. I created the shell users in Debian & set their home directory as their mailbox.
My question is after I added about 40 users, I realized that anyone can simply Telnet to my mail server on port 25 and compose a message and say they're someone else:
telnet my.mailserver.tld 25
Hey! You're a fat pig & I quit!
Message queued as S7439OP32
So I can send that from any PC on the domain and claim that I'm 'Bob' when in fact I'm not. This seems like a really big issue for security & authenticity for Postfix / MTA. How can I resolve this issue and or prevent it from happening?