Email Protocols is an essential part of virtually every organization. Let’s first discuss mail retrieval protocols, POP3 and IMAP, Then cover mail transmission with SMTP.
In Email protocols The Post Office Protocol is a legacy email retrieval system. The most current version is 3, which is why it is most often referred to as POP3. A POP3 server listens on TCP port 110. An email client will connect to a POP3 server, download any emails, store them on the client’s PC, then delete them from the server.
Most clients have the ability to leave a copy on the server, but this isn’t the default behavior. POP3 does have a Secure option, whereby communication can be encrypted using TLS or SSL. POP3S runs on TCP port 995. There are several authentication methods supported by the POP3 extension mechanism, such as SASL, a Kerberized verison, and APOP, that uses MD5.
Due to its advanced features and flexibility, the Internet Message Access Protocol has supplanted POP3 for email retrieval.
IMAP was designed with the idea that a user will access their email from multiple clients. It’s for this reason that by default email is left on the server. The newest version as of this writing was IMAP Version 4.1, and was published in 2003 as RFC3501.
Standard IMAP is available via TCP port 143, where the secure version IMAPS, is available via TCP port 993. IMAP tends to stay connected to the server, so as messages arrive, they are quickly delivered. POP’s offline nature means it will periodically check. POP allows only a single client to access a mailbox at a time, where IMAP specifically allows multiple clients simultaneous access. IMAP tracks message states also, so each email can be tagged as read, replied to, or deleted.
These flags are stored Server-Side, so multiple clients can see the state of messages. IMAP supports Server-Side Searches. This allows clients to pass certain criteria to the server for searching. The Simple Mail Transfer Protocol was originally defined in 1983, but more recently revised in 2003 by RFC 5321. It defaults to TCP port 25. As the name implies, it is generally used by mail servers to both send and receive email from outside of their organization.
An email client that uses POP or IMAP will also send using SMTP. Clients that use proprietary extensions like Outlook MAPI don’t generally use SMTP to send email, though the server will still transfer email via this method.SMTP was originally developed to carry standard ASCII text, not files. So Multipurpose Internet Mail Extensions were created to support binary files.Some networks will employ an SMTP relay server.
This is usually used to act as a spam protection mechanism. Spam bots are often thwarted by only allowing SMTP to be sent from a relay server. SMTP Relays often employ some basic mail scanning to further prevent spam.Believe it or not, SMTP functionality can be easy to test. Using telnet, you can connect to an SMTP server and actually send an email via plain text commands. Since email has become so necessary, this topic could not be more important.
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