It's the “signature” (free text, but people use it for information, address, phone number, disclaimers and what not). It appears under the “tearline”, that consists of two dashes and a space (“-- ”).
But how is a website supposted to get access to user's files in his homedir?
It's not. Sender's email client will/can insert their ~/.signature file at the end of each outgoing email. When such an email is read in email.ml, the signature is mangled in the way I originally described.
Ah, that's why I was confused: I did not understand why would a website has anything to do with files on a POSIXish machine. Thanks.