SMTP- Simple Mail Transfer Protocol

What is SMTP?

Simple Mail Transfer Protocol for electronic message which works on Port-25

https://tools.ietf.org/html/rfc821  (1982)

https://www.ietf.org/rfc/rfc5321.txt (1998)

How it works ?

If you want to send email out of your organization you need to have MX record of particular receivers listed on your dns.

Now what happen when we send email?

It goes to our SMTP Server and if we are sending email outside SMTP server queries to DNS if it has MX record for receivers.

DNS provide necessary information to SMTP server and here actual conversation start between 2 parties.

     Send SMTP Server connect Receivers SMTP Server on Port 25

For example (Mail. *****.Com Port 25)

Receiver SMTP Server: Send acknowledge by sending ELHO command.

Sender SMTP Server: - Mail from 123@XYZ.com

Receiver SMTP Server: - 250 (OK)

Sender SMTP Server: - Recipient Mail ABC@*.com

Receiver SMTP Server: - 250 (OK)

Sender SMTP Server: - DATA

Receiver SMTP Server: - 354 (Start mailing input)

Sender SMTP Server: - Actual Message body goes here and when message ends it ends with . and new line.

.

  (After . and blank line receiver understand message text is complete and acknowledge. 

Receiver SMTP Server: -250 (Queued for message delivery)

  

MX Record: - It is type of resource record in DNS which specifies a mail server responsible for accepting email messages on behalf of a recipient's domain.