To change the Outgoing Mail Server port from 25 to 587 follow the steps below:
1. Open Thunderbird.

2. Select the Tools menu.

3. Choose Account Settings...

4. When the new Account Settings window opens, click Outgoing Server (SMTP) on the left.

5. Click the Edit... button.

6. Then change the field that says Port: to 587. Click OK and then restart Thunderbird.




