Unable to use Thunderbird

Thunderbird email client mails blocked by popup. I cannot access mails on Tbird because a pop-up appears saying I am about to alter how Tbird identifies this site. I have more than 10 accounts set up via my BlueHost Hosting system.The same popup occurs for all of them.  I can access all the mails using web mail rather than the Tbird client. I haven't any other client systems to access the mails.
The system has Spanish language and I attach a screen grab. The translation is: You are about to alter how Tbird identifies this site. It gives the server address mail.domainname.es:143
State of certificate
- This site is trying to identify itself with invalid information
The popup invites to accept security exception. If I press this it pops up again with a different domain name,

I have Avast Internet Security installed. If I look in detail Avast protections appear in what it calls Basic Shields.
Details of certificates:
Certificate of Server SSL
Domain Control Validated
Emited by
avast! Web/Mail Shield Root
generated by avast! antivirus for SSL/TLS scanning
It is beginning to look as if Avast is guilty overall. What would recommended cure be please?
Michael GunnerDirectorAsked:
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Jackie ManIT ManagerCommented:
Uninstall Avast Internet Security, restart computer and try again.
Michael GunnerDirectorAuthor Commented:
Thank you. Your opinion backs up what I had thought. I am in the process of doing the uninstall (which is more complicated than I thought).
Davis McCarnOwnerCommented:
Apparently, Avast became a certificate authority service in 2016 and your host is on a long list of places that Avast is not happy with.  If uninstalling doesn't fix it, read this article on how to add an exception for your mail server:
https://support.mozilla.org/en-US/kb/add-security-exception
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MereteCommented:
Just a side share,
 I really like the fact that Thunderbird would lookup and auto set the all the necessary protocols for the email server and it was these I used / copied into the ( now defunct ) windows Live Email client and got it working again so thanks to Thunderbird I am  still using  Windows Live Email client years later.
Thunderbird definitely needs all access  very good email client either way.
Michael GunnerDirectorAuthor Commented:
I uninstalled Avast and unfortunately the problem remains with Thunderbird. I will look at the other steps suggested here but it will take me some time because I need mail and have to use a web mail which is cumbersome. I will not close the question as I have no answer that works yet. Thank you for your help.,
MereteCommented:
Look under menu path Edit  or Tools-> Account Settings->Server Settings->Server name. The entry in that field should read mail.ISPName.com
 Try changing it to this value and see if that resolves the issue.
also check your Outgoing Server ( SMTP) lists your ISPname.mail-smtp.ISP.com (Default)
Michael GunnerDirectorAuthor Commented:
I have not yet tried the last suggestion by Merete. I will get round to it and thank you for your help, Meanwhile I am using webmail. I want to keep the case open while I have no tried and tested solution.
Michael GunnerDirectorAuthor Commented:
I believe I have solved this problem. My mail accounts were not "clean" in that I had a deleted account with a domain which was still active in the DNS server, and pointing to my email server with Bluehost. Every time I started Thunderbird this phantom email account would come up with an error message - Unable to connect. I have now removed the Name Server reference and so far I have not had this annoying (and very time-consuming) problem. I don't know enough about emails to be sure but it seems all right now. So nothing to do with Avast which I put back. Thank you for your help everybody!

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Michael GunnerDirectorAuthor Commented:
There were other things suggested to try but I thought I really should remove a persistaent error which I thought was unrelated. Now it all workd but if th eproblem comes back I still have a couple of things to try which hve been suggested here. Many thanks
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