There's often quite a bit of confusion around the different terms SSL, TLS and STARTTLS.
SSL and TLS both provide a way to encrypt a communication channel between two computers (e.g. your computer and our server). TLS is the successor to SSL and the terms SSL and TLS are used interchangeably unless you're referring to a specific version of the protocol.
STARTTLS is a way to take an existing insecure connection and upgrade it to a secure connection using SSL/TLS. Note that despite having TLS in the name, STARTTLS doesn't mean you have to use TLS, you can use SSL.
Version numbering is inconsistent between SSL and TLS versions. When TLS took over from SSL as the preferred protocol name, it began a new version number, and also began using sub-versions. So the ordering of protocols in terms of oldest to newest is: SSL v2, SSL v3, TLS v1.0, TLS v1.1, TLS v1.2.
When you connect to an SSL/TLS encrypted port, or use STARTTLS to upgrade an existing connection, both sides will negotiate which protocol and which version to use based on what has been configured in the software and what each side supports.
Support for SSL/TLS is virtually universal these days, however which versions are supported is variable. Pretty much everything supports SSL v3 (except a few very old Palm Treo devices as we discovered). Most things support TLS v1.0. As at May 2012, support for TLS v1.1 and TLS v1.2 is more limited.
One significant complicating factor is that some email software incorrectly uses the term TLS when they should have used STARTTLS. Older versions of Thunderbird in particular used "TLS" to mean "enforce use of STARTTLS to upgrade the connection, and fail if STARTTLS is not supported" and "TLS, if available" to mean "use STARTTLS to upgrade the connection if the server advertises support for it, otherwise just use an insecure connection".
The above is particularly problematic when combined with having to configure a port number for each protocol.
To add security to some existing protocols (e.g. IMAP, POP, etc.), it was decided to just add SSL/TLS encryption as a layer underneath the existing protocol. However, to distinguish that software should talk the SSL/TLS encrypted version of the protocol rather than the plaintext one, a different port number was used for each protocol. So you have:
143, but SSL/TLS encrypted IMAP uses port
110, but SSL/TLS encrypted POP uses port
25, but SSL/TLS encrypted SMTP uses port
At some point, it was decided that having 2 ports for every protocol was wasteful, and instead you should have 1 port that starts off as plaintext, but the client can upgrade the connection to an SSL/TLS encrypted one. This is what STARTTLS was created to do.
There were a few problems with this though. There was already existing software that used the alternate port numbers with pure SSL/TLS connections. Client software can be very long lived, so you can't just disable the encrypted ports until all software has been upgraded.
Mechanisms were added to each protocol to tell clients that the plaintext protocol supported upgrading to SSL/TLS (i.e. STARTTLS), and that they should not attempt to log in without doing the STARTTLS upgrade. This created two unfortunate situations:
Both of these problems resulted in significant compatibility issues with existing clients, and so most system administrators continued to just use plaintext connections on one port number, and encrypted connections on a separate port number.
This has now basically become the de facto standard that everyone uses. IMAP SSL/TLS encrypted over port
993 or POP SSL/TLS encrypted over port
995. Many sites (including FastMail) now disable plain IMAP (port
143) and plain POP (port
110) altogether so people must use an SSL/TLS encrypted connection. By disabling ports
110, this removes completely STARTTLS as even an option for IMAP/POP connections.
The one real exception to the above is SMTP. However that's for a different reason again. Most email software used SMTP on port
25 to submit messages to the email server for onward transmission to the destination. However, SMTP was originally designed for transfer, not submission. So yet another port (
587) was defined for message submission. Although port
587 doesn't mandate requiring STARTTLS, the use of port
587 became popular around the same time as the realisation that SSL/TLS encryption of communications between clients and servers was an important security and privacy issue.
The result is that in most cases, systems that offer message submission over port
587 require clients to use STARTLS to upgrade the connection and also require a username and password to authenticate. There has been an added benefit to this approach as well. By moving users away from using port
25 for email submission, ISPs are now able to block outgoing port
25 connections from users' computers, which were a significant source of spam due to infection with spam-sending viruses.
Currently, things seem relatively randomly split between people using SMTP SSL/TLS encrypted over port
465, and people using SMTP with STARTTLS upgrading over port