Port 25 TCP
SMTP, used for sending email to a mail server.
SMTP stands for Simple Mail Transfer Protocol, and is defined as an official internet standard in RFC 821 and RFC 5321.
eMail servers transfer mail between each typically using SMTP as the transporting protocol, similarly when eMail clients on a user’s computer or device send email they will use SMTP, however when a client receives email they will normally use either IMAP or POP3.
The protocol will use a TCP port 25 when sending from one server to another, however when a client is sending email it will also use TCP port 587 (and even sometimes a non-standard legacy port of 465)
It is possible to secure SMTP with SSL, in which case it becomes the SMTPs protocol, in a security conscious world this is becoming more widely used.
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