Understanding Email Part 3 of 5 – Email Protocols
By Steve Swaney
The related video blog can be found on YouTube here.
This is Part 3 of a five part series on understanding how email works. This is based on a presentation I gave at Florida State University to a graduate level class for Computer Systems Administrators. This presentation is about email protocols.
There are four main email protocols:
- POP: the Post O ffice Protocol
- IMAP: the Internet Message Access Protocol
- SMTP: Simple Mail Transfer Protocol
- MAPI: the Messaging Application Programming Interface as defined by Microsoft
POP and IMAP are used to transfer email from a mail hub to a client mail tool while the SMTP protocol is used to transfer mail from a client mail to a mail hub and between mail hubs on the internet. MAPI is used to transfer mail and other data between Microsoft’s Outlook email Client and Microsoft Exchange servers. Note that Exchange servers also support POP, IMAP and SMTP Protocols to allow them to transfer email to non-Microsoft email clients and mail servers.
The first RFC to define how a MUA or client mail tool could retrieve and modify emails from a central mail Store or email server was POP. the Post Office Protocol. the first POP RFC was released in 1984 and, with modifications, POP is still in use today.
But the Internet Message Access Protocol, released in 1988 and updated many time ssince, provides more features than POP. The main advantage IMAP has over POP is that IMAP allows many devices, like your phone, computer and tablet to share the same view of a central Mail Store, so IMAP it should probably be the protocol to select if you need to synchronize email across multiple devices.
POP and IMAP are the protocols commonly used to fetch mail from your email server to your email client on your computer or mobile device. But SMTP is the protocol most often used to send mail that you have composed on your email client to your email server, and then from your email server to the recipient’s email server.
In fact while there are several possible protocols that can be used to send or fetch email between mail clients and mail servers, the Simple Mail Transfer Protocol is always used to send mail from mailhub to mailhub if the message travels across the Internet. It is the SMTP protocol that made universal email exchange possible.
As English is the universal language for pilots and air controllers SMTP is the universal language for Internet postmasters and email traffic.
The SMTP Protocol was first defined by RFC 821 in 1982, before the Internet as we know it even existed. Currently RFC 2821 defines the basic protocol for the Internet electronic mail transport, while RFC 5321 consolidates, updates, and clarifies many earlier RFCs. It also covers the SMTP extension mechanisms and best practices for the contemporary Internet.
And finally there are Microsoft’s MAPI protocols. If you use Microsoft Outlook as your email client and Outlook is configured to use Microsoft Exchange for it’s email server, Outlook and Exchange are transferring data using Microsoft’s Messaging Application Programing Interface. Also any third party application that works with Exchange or Outlook will use MAPI Protocols to connect. But even the Exchange Server typically uses the SMTP Protocol to transfer email to external sites.
More information on these protocols can easily be obtained by searching for the protocol name in Wikipedia. And the RFCs can be found by searching Google for the RFC nomber. The RFCs are actually fairly easy to understand, even without advanced technical computer skills.