I use Thunderbird as an email client all the time. It works fine until today after I updated Kloxo-MR 7.0.0.c-2017060601. Now I can't send out emails. I get this error.
Sending of the message failed.
The message could not be sent using Outgoing server (SMTP) mail.domain.com for an unknown
reason. Please verify that your Outgoing server (SMTP) settings are correct and try again.