@SqNetP do you have an IT support person to call upon for help?
I suspect that if we use an analogy here it might help a little bit, for the SSL (i.e TLS) side and your Thunderbird certificate mismatch.
Say you have 2 houses and want one key to open both houses, you need both houses' locks to be keyed the same basically a copy of each other.
Now if we name the houses
- Squarespace Inc.
- Netcetera Ltd.
Then presently your 1. Squarespace Inc. is keyed fine for your one key.
However your 2. Netcetera Ltd. is NOT keyed for your one key.
So you need a locksmith (i.e. IT support person) to rekey 2. Netcetera Ltd. to use your one key.
They you do that by copying the Private Key and Certificate from 1. Squarespace Inc. to 2. Netcetera Ltd. so they both can provide the same response. The one key (i.e. a SSL Certificate) needs to have the DNS names of both systems in it.
House 1. A pamelabuchan.co.uk 3600 198.49.23.144
House 2. MX pamelabuchan.co.uk 3600 10 mail.pamelabuchan.co.uk. (81.27.105.135)