Ah, the ever informative "Server error" error.

Sorry to hear your issue was caused by another changing the goal posts - Always frustrating when that happens, especially when you have a contract for service provision.

I think that would be a deal-breaker for me, if they don't fix it. I mean, if that was the original reason you chose to purchase services, or the change has stopped the service being usable to you, you should tell them their hosting service holds no value for you any more and then just drop them like a hot potato.

Some country's legislation favours the company, so you should check if you are outside the UK. I believe changing the service provision breaks any prior agreement you made with them (unless that's also in the contract) so you can say no thank you and find someone better.

Could you ask them for specific account connection parameters which they DO support? (i.e. IMAP, SSL, StartTLS, password encryption, ports, etc...)

Chances are you could get Thunderbird to use those instead.

Otherwise, I would recommend going with a better service provider, to be honest. I mean, Thunderbird is not uncommon and doesn't do anything different to other mainstream e-mail clients so there is no reason Thunderbird should be specifically blocked... Unless they're picking on Thunderbird and want you to use a web client.

I would switch hosting provider if they can't give you access to your own e-mails.

I run my own e-mail server and I have no issues lol

However, it's not something I would recommend to the uninitiated - it's time-consuming to get it done properly.

What are your settings, and are you using an app password?

https://www.rogers.com/customer/support/article/email-setup