1. Start Thunderbird and click on "Create a New Account." If it's the first time you open Thunderbird, you can click directly on "Skip this and use my existing email."
2. Input your name, email address and password then click "Continue." Thunderbird will try to detect the email settings.
3. Click on "Manual Config" even if Thunderbird found your email settings.
4. You will need to edit your connection settings.
   
   For Incoming using IMAP  - for Server hostname you can use either "mail.yourdomain.com"  or "s#.fused.com" replacing # with your server number. Your server number is in your Account creation email.
   
   Port: 993 with SSL/TLS and Normal password selected for Authentication.
   
   For Outgoing using SMTP - for Server hostname use the same you used for IMAP, either "mail.yourdomain.com or "s#.fused.com" replacing # with your server number.
   
   Port: 25 or 587 with STARTTLS/TLS and Normal password selected for Authentication.
   
   Make sure you always use your full email address for Username. eg: john@yourdomain.com
5. Confirm security exception if you set the hostname to "mail.yourdomain.com." You won't be prompted for this if you set it to "s#.fused.com"
   You will see a warning message again the first time you will send an email if the hostname for SMTP was set to "mail.yourdomain.com"

Check the attached images for demo setup.