1. Open Tools, and click Account Settings. Then, click Account Actions and select Add Mail Account...

1. Fill in the following information:

-Your Name: Display name you wish email recipients to see

-Email address: Your Pobox Address

-Password: Your Pobox Password

Be sure to check "Remember password" Then click Continue.

1. Thunderbird will attempt to find the proper mail servers here, but to ensure you have the correct settings, please click the button on the top right of this pane, which will be either "Edit", or "Stop" (then click "Edit").

1. Thunderbird 3 will continue to try to "guess" the correct Incoming Mail settings as you are filling them in, so we will be filling most of these in within the Manual Setup. Before we do so, however, some of this pane must be filled in to properly set up the account. Start with filling in all of the Outgoing line:

Outgoing: smtp.pobox.com

SMTP

Port: 465

SSL/TLS  (This is important!  Using STARTTLS will cause errors and unusual disconnects.)

Then make sure the Incoming line has been switched from "POP" to "IMAP", and click "Manual Setup".

1. Make sure that the following information is filled in:

Server Type: IMAP Mail Server

Server Name: mail.pobox.com

Port: 993

User Name: Your Pobox Address

Connection security: SSL/TLS  (this is important!  Using STARTTLS will cause errors and unusual disconnects.)

1. Click OK, and OK again to the pop-up warning. You have finished setting up Thunderbird 3 to send and receive your Pobox mail.

(NOTE: Thunderbird 3 seem sluggish to you? You aren't the only one. Click that link to go to Mozilla's community-powered support page for this issue. There, you can compare your program's performance with other Thunderbird users' slowness reports, and try a few possible fixes at your own risk.)

(You may also want to refer to this page here for another configuration change. Thanks to Wayno for the details!)