Thunderbird not sending mail

I am sure that I have the correct settings in Thunderbird, but it will not send mails, returning the error message:
"Sending of the message failed.
The message could not be sent because connecting to Outgoing server (SMTP) smtp.zoho.com failed. The server may be unavailable or is refusing SMTP connections. Please verify that your Outgoing server (SMTP) settings are correct and try again."
I have removed the account and run setup again using recommended settings, retested and Thunderbird couldn't verify those settings.
I went with them anyway, and Thunderbird seems to be checking emails correctly, but won't send.
I believe this problem may have started when I installed the Zoho email app on my phone.
I see there are many posts about similar problems. Is there a solution?