TLDR
Your mail client contacts your email server over SMTP.
Your email server contacts their email server over SMTP.
Their email server then stores the message.
Their email client retrieves the email over either POP or IMAP.

Email

The email system composes of, a lot of acronyms.

Note: A mail server is a machine which embeds a mailbox, an MTA and an MDA, plus the respective server software for SMTP, POP and IMAP

Mail User Agent (MUA)

The MUA is your email client, where you compose, send and receive emails.

Your MUA is responsible for fetching your mailbox (either through IMAP or POP) to retrieve emails.
To send emails, your MUA contacts your MTA via SMTP.

Mail Transport Agent (MTA)

The MTA is a software that handles the delivery of your email to the recipient’s MTA.

Your MUA first contacts your local MTA over the SMTP protocol.
Your MTA then contacts the recipient’s MTA, where the mail is then passed onto their MDA.

This software is responsible for (trying to) sending/receiving emails even when your/their computer is offline.

Mail Delivery Agent (MDA)

The MDA handles the reception of emails, where they are sorted and placed into a local mailbox.

When your MUA wants a copy of the emails, it will contact either a POP Server, or an IMAP Server. These servers are responsible for accessing the local mailbox.

Protocols

Simple Mail Transfer Protocol (SMTP)

The SMTP protocol is used to SEND emails, rather than to receive

  • RFC5321
  • Persistent connection (Connection remains open until closed)
  • 7-bit ASCII
  • \r\n.\r\n to signify the end of message

Post Office Protocol (POP)

The POP protocol is used to RECEIVE emails, rather than to send

  • RFC1939
  • One way synchronisation (does not update the “read” property of an email)

Internal Mail Access Protocol (IMAP)

The IMAP protocol is used to RECEIVE emails, rather than to send

  • RFC1730
  • Two way synchronisation (can mark opened emails as “read”)
  • Better than POP

Why do we have the sender’s mail server

In the case that the receiver’s mail server is down, the sender’s mail server will handle the delivery of the message.
In this way, the sender’s computer can be turned off / disconnected.

(Ahem, timezones!)

Phishing

  • Spear Phishing
  • Clone Phishing