Courier does not have native SSL support at this time. You can use an SSL tunnel program such as Stunnel, which is free.

To use stunnel do the following:

1) download the windows installer from http://www.stunnel.org/

Stunnel 4.14

Run the installation, which will install the program and required library files to Program Files\Stunnel\ by default.

2) Edit the "Run Stunnel" Start Menu shortcut (Start | Programs | Stunnel | Run Stunnel) by right clicking on it and selecting Properties.

Modify the Target to look like this:
"C:\Program Files\Stunnel\stunnel.exe" stunnel.conf

3) create a text file and named it 'stunnel.conf' in the same directory as the program files, and put the following lines in it:

----starting with next line----
# GLOBAL OPTIONS

client = yes

output = stunnel-log.txt
debug=4
taskbar = yes
# SERVICE-LEVEL OPTIONS

[POP3 (ISP name)]
accept = 127.0.0.1:108
connect = myISPs.POP3server.net:995

[SMTP (ISP name)]
accept = 127.0.0.1:107
connect = myISPs.SMTPserver.net:465
----ending with previous line----

Change the server names (before colons), and perhaps ports (after colons), to be correct for your ISP. (Port 995 is the "standard" port for retrieving mail via a secure POP connection, and 465 is the "standard" port for sending via a secure SMTP connection, but your ISP may use different ports). Change the ISP name for your personal reference (this does not affect program performance). Save and exit.

For example, if you are using Comcast and want to configure Stunnel to let you connect to their servers using SSL, your Service Level Options would read:

[POP3 (Comcast)]
accept = 127.0.0.1:108
connect = mail.comcast.net:995

[SMTP (Comcast)]
accept = 127.0.0.1:107
connect = smtp.comcast.net:465

3b) If you have two email providers requiring SSL you can add a second set of services like so:

[POP3 (2nd ISP name)]
accept = 127.0.0.1:106
connect = my2ndISPs.POP3server.net:995

[SMTP (2nd ISP name)]
accept = 127.0.0.1:105
connect = my2ndISPs.SMTPserver.net:465

Each email provider requires its own two ports (e.g. 108+107, 106+105), but 2 email accounts at a single provider can share the same pair of ports. However, if an email provider only uses SSL for one of smtp or pop3, you only need one of the two service sections.

Once again, change the server names (before colons) to be correct for your ISP. Change ISP name too. Save and exit.

4) Put a copy of the "Run Stunnel" shortcut into your Start Menu's Programs/Startup folder [normally, by dragging shortcut over Start button, over Programs, over Startup, then drop it in Startup list.]

5) Edit your Courier Account Properties, in the Mail Server tab, to change your mail servers so that they connect to Stunnel instead of directly to your ISP.

Incoming Mail Server:
Port: 108
Server Name: 127.0.0.1

Outgoing Mail Server:
Port: 107
Server Name: 127.0.0.1

(Your "User ID" remains the same).

If an email provider only uses SSL for one of smtp or pop3, you only need to change one of the two mail servers. If you set up more than one email provider for SSL, change each account's properties to the corresponding ports.

Now when you check or send email Courier will communicate with stunnel, which will connect to your ISP's mail servers via SSL. The shortcut in Startup will rerun stunnel each time you reboot. You should just double-click on the shortcut in the program folder to run stunnel the first time.