#install mail tools $apt-get install mailutils #install telnet client $apt-get install telnet
#telnet some_ip port $telnet x.x.x.x 25
If you get an output, it's means the smtp respond !
$mail -s "Test Subject" firstname.lastname@example.org < /dev/null
#telnet x.x.x.x 587 or 25 HELO < you start conversation 250 OK < server lets you proceed MAIL FROM: <email@example.com> 250 OK < server is fine with this sender RCPT TO: <firstname.lastname@example.org> 250 OK < server is fine with recipient DATA < you are going to send email body 250 OK < server is fine with that - proceed Here you send your message . < need to close the DATA by the dot ! 250 OK < server accepted message QUIT