$ nc mxs.mail.ru 25
220 mxs.mail.ru ESMTP ready
HELO hdjdj
250 mxs.mail.ru
MAIL FROM:sheisi@mail.ru
250 2.0.0 OK
RCPT TO: djsjekwk@mail.ru
550 5.1.3 Bad destination mailbox address syntax: invalid mailbox. Local mailbox sheisi@mail.ru is unavailable: Ill-formatted e-mail address