Quoting rfc 2821:
The MAIL command (or the obsolete SEND, SOML, or SAML commands)
begins a mail transaction. Once started, a mail transaction consists
of a transaction beginning command, one or more RCPT commands, and a
DATA command, in that order. A mail transaction may be aborted by
the RSET (or a new EHLO) command. There may be zero or more
transactions in a session. MAIL (or SEND, SOML, or SAML) MUST NOT be
sent if a mail transaction is already open, i.e., it should be sent
only if no mail transaction had been started in the session, or it
the previous one successfully concluded with a successful DATA
command, or if the previous one was aborted with a RSET.
SmtpServer#process_mail_from? currently violates the last paragraph, rejecting a second MAIL FROM command even after DATA succeded.
