- [Instructor] Email's an essential part
of virtually every organization.
Let's first discuss mail retrieval protocols,
POP and IMAP, then cover mail transmission with SMTP.
The Post Office Protocol is a legacy email retrieval system.
The most current version is three,
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 new 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 mechanisms,
such as SASL, a Kerberos version and APOP that uses MD5.
Due to its advanced features and flexibility,
the Internet Message Access Protocol
has supplanted POP3 for mail 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.
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're quickly delivered.
POP allows only a single client to access
a mailbox at a time where IMAP specifically
allows multiple clients and simultaneous access.
IMAP tracks message states also.
So each email can be tagged as read, replied to, or deleted.
These flags are stored server-sides
so multiple clients can see the state of messages.
IMAP supports server-side searches also.
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 the organization.
An email client that uses POP or IMAP
might also send using SMTP, but it's not very likely.
Clients that use proprietary extensions like Outlook's 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.
The concept is only this single server
is allowed to send SMTP message outbound from the network.
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 employs some basic mail scanning
to further prevent spam.
Believe it or not, basic 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.