Servers run on port 25.  
This protocol was first defined surprisingly late in RFC 821.  Servers run on port 25.  Without some provision for data transparency, the character sequence <CRLF>.<CRLF> ends its text.  Its command keywords are four letters long; sessions begin with either the keyword EHLO or the keyword HELO, depending on whether extended features such as those used for sending large MIME objects are to be used.  Usually returning the command RCPT for successfully sent data, the most notorious server for it is sendmail.  FTP, identify this "simple" protocol used to send mail on the Internet.
This protocol was first defined surprisingly late in RFC 821.  
SMTP or simple mail transfer protocol