Simple mail transfer protocol

Simple Mail Transfer Protocol is used for transmitting e-mail over the internet using the IP protocol. Clients(or users) uses some kind of user interface(MUA – Mail User Agent) to transmit and/or read their e-mail. This can be done by webmail, outlook or any other mail client(application).

The SMTP protocol is used to transfer email from mail server A to mail server B(or more). Users can read their email by using other protocols like IMAP or POP3*, but those protocols are out of scope in this article.

When a client submits an email, it will go to an “mail server” which will forward the email over the IP protocol to another mail server and waits till a user picks the email up from the mail spool using a protocol for incoming mail like POP3* or IMAP.

STMP uses TCP port 25

Schematic overview – How SMTP works

* Current version at the moment of writing this article

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