1 - Set up your Gmail account in Thunderbird, if you haven't already
2 - Go to Tools > Account settings > Outgoing Server (SMTP)
Enter your SMTP credentials (found in the Templ panel, under Email > SMTP Settings)
Select "SSL/TLS" as connection security.
(SMTP password will be entered later)
3 - Tools > Account settings > Select your Gmail account in the left tab > Manage Identities
Enter the name and email address that you want to use when you send emails.
4 - Send a test email
Make sure to select the identity you just configured as "From"
When send the email, you will be asked to enter your SMTP password. Enter it and your email should be sent successfully using your own address.