SMTP unknown error can't send emails

Setup Virtualmin with the main domain. Email sends and receives no problem.

Added several other domains to the system. I can receive email, but not send.

My situation: (receive/send) (receive/ NO SEND) (receive/ NO SEND) (receive/ NO SEND)

I cannot find any of my Postfix log files, to see what the error is.

The only message I get is in Thunderbird, see below:

“Sending of the message failed. The message could not be sent using Outgoing server (SMTP) for an unknown reason. Please verify that your Outgoing server (SMTP) settings are correct and try again.”

Please help I have to answer the emails!!!

Reboot my entire server everything is working!