Sure, this is on my worklist. But as I was able to understand, this is not a simple task and dangerous - risk to end with non-working server.
It isn't that hard, but takes a bit of patience. That is because of upstream choices, not ours.
So, why haven't you been testing and planning like others? This has been ongoing for a long time?
BTW, IMHO v10 was released rather recently.
It was. We released when we felt ready after extensive testing. We have been around to assist with queries and issues.
We know how important stability is so we don't do it on a whim.
Note that it has been pretty stable since the first Beta, and I have run one server since fairly early Alpha without any real issues and just following bugs and updates, and being careful. YMMV.
So, making previous(!) version EOL seems to me a bit non-logical.
If you had kept up to date you would realise it was perfectly logical as the underlying OS was EOL - not our choice - so you can take it up with CentOS and RedHat if you think it illogical.
Anyway, I can't do this TODAY without even option to reverse to working server as the current server is also not fully working...
This does make me wonder about the response from say Microsoft if you had an issue with DOS, or Windows 98, or say Ubuntu with say 8.04 or even 15.10 ?
So, please, help me to solve this, PLEASE!!!
As you haven't really give that much info it is hard to tell. You report of 'reading on Horde is OK. Sending via Thunderbird is OK'. They are not the same.
Test you can get to
https://your.server.com with a browser and the certificate is OK.
Test you can send AND receive on Horde.
If they are OK you probably have a client issue.
Beyond that.....
I promise to update to v10 soon...
If you fix this, I wonder when 'soon' will be?
From experience it usually means 'ah it works now so I'll leave it for for the minute'. Until the next time. Rinse and repeat.
Sorry to be cynical or appear harsh, but if you run a server then you should be keeping up to date. Everyone has had lots of time to know that v9 was going to be EOL, and lots of time to prepare and test. Some even helped us to do that.
Now is not a great time to be trying to fix your EOL server. And that is why the v9 forum was locked.