Can anyone help with this Thunderbird problem?

Running TB 3.1.6 on Win7. Will work fine, sending and receiving, then fail to send. Error message is:

An error occurred while sending mail. The mail server responded: Sorry, that domain isn't in my list of allowed rcphosts, no valid cert for gatewaying (#5.7.1). Please check the message recipient [email address] and try again.

Repeated tries will sometimes get the message through. Other times, bouncing TB will fix the problem and allow the message to be sent and, at other times, a reboot is required.