An update on this situation. Since I last posted I have taken my laptop to various different locations and been able to successfully send messages via the gridhost server from them (the library, supermarket, my sister's house, the computer shop etc.) - just not from home. So it's not a problem at all with T'bird, but something to do with my home connection. Following advice from the VM forum I have run a tracert here at home which shows there is no problem getting to the gridhost servers. They suggested changing my hub settings to modem only which I did, and I was able to send messages from the outgoing server at gridhost without any problems, so apparently the answer seems to be that gridhost is blocking my public IP address. I can now chase this up with them. I feel that there is light at the end of the tunnel!

I haven't done the openssl command thing yet - do I still need to try that?

I haven't done the openssl command thing yet - do I still need to try that?

Probably not.

Gridhost were blocking my IP, apparently due to too many login failures. This makes no sense to me as I always logged in without any problems until suddenly I couldn't, so it will remain a mystery. So now I am back to normal functioning. At least I have a much better idea of what diagnostics to run and who to turn to in case anything similar happens in the future. Again many thanks to everyone, have a good day!

Good. Can you then mark the thread as 'Solved' please? Thank you.

just had the same problem on my mac pro and the new sierra operating system. make sure you don't op for the two security password option. this will disable your thunderbird and tell you you are not putting in the proper password.