Originally Posted by JimmyStephans

I use Thunderbird for 16 accounts at the same time. Its a breeze.

If it can't reach the server it has zero to do with Thunderbird. 3 main options:

1- If new account, you have something set incorrectly. Maybe server calls for TSL or different port, etc.

2- If older account, maybe the mail server that sends the mail is simply down. Happens often.

3- If you are away from your normal location (say in a hotel), those places often block the main email port (25) to avoid people doing spamming on their networks.