I have setup GMail to use another email account I have (non-Google), so when I send emails from GMails web UI, I can choose to send from that other domain. How do I add such an option to Thunderbird? That is, I would like to use GMails SMTP server, but send messages using my other account.
Thunderbird allows you to use different identities for an email account. You can do this in order to use different formatting, signatures, reply-to addresses, templates, encryption, etc for different circumstances. For instance, if you have another email domain attached to your GMail account, you can add another identity for that domain. You can then choose which identity to send from when composing a message, as if you had several email accounts added to Thunderbird.
It's quite easy to configure:
In Thunderbird, if you click Edit > Account Settings, you will be given a window which allows you to make all sorts of changes. From the menu on the left, choose "Outgoing Server (SMTP)," and then on the right side you'll see the current SMTP servers you have set up. Click the "Add" button to, um, add a new SMTP server:
Once you have entered the new information into the smaller window, click OK. You can now highlight this server in the Outgoing Server (SMTP) Settings window, and choose the "Set Default" button on the right. Choose the OK button at the bottom, and then close Thunderbird and reopen it, and when you send e-mail it will use the account you have chosen for your SMTP server.
Your POP or IMAP server will receive mail, and your SMTP server will send it.