Port 25 is used by SMTP, which is used to send email. While POP3 (110), and IMAP (143) are instead used to receive email.
Simple Mail Transfer Protocol (SMTP) is an Internet standard for email transmission. First defined by RFC 821 in 1982, it was updated in 2008 with Extended SMTP additions by RFC 5321; which is the protocol in widespread use today.
Mail servers and other mail transfer agents use SMTP to send and receive mail messages on TCP port 25. Although proprietary systems such as Microsoft Exchange and IBM Notes and webmail systems such as Outlook.com, Gmail and Yahoo! …