GMail being blocked on Thunderbird by Avast Free

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GMail is being blocked on Thunderbird by Avast Free Edition.
It started by popping up notices that Googles' certificate was invalid, giving an option to approve and make the exception permanent, which I did every time the email was retrieved or sent.
Then new messages just stopped coming in, I found the instructions from Avast to add the MailShield Security Certificate and installed it properly - made no difference.
Went to Avast Settings and turned off the Mail Shield and new messages came pouring in.
Is there an option to have Mail Shield work, or do I just need to leave it turned off?

Avast Free Edition
Mozilla Thunderbird eMail client
Windows XP SP3

[Eddy]

The problem is that you are using XP.
It can't handle the new(er) certificates.

https://blog.mozilla.org/security/2014/09/23/phasing-out-certificates-with-sha-1-based-signature-algorithms/

[Pondus]

or do I just need to leave it turned off?

Google protect all Gmail accounts using Postini spam/virus filter > Used

Postini > https://en.m.wikipedia.org/wiki/Postini

2012 > Google Postini Email services will be shutting down, new encryption service appears to be on the way
https://9to5google.com/2012/08/21/google-postini-email-services-will-be-shutting-down-new-encryption-service-appears-to-be-on-the-way/

2015 > Google starts applying artificial intelligence to Gmail spam filters
http://www.digitaltrends.com/android/google-starts-applying-artificial-intelligence-to-gmail-spam-filters/

http://www.pcworld.com/article/2995524/security/google-yahoo-tighten-spam-filtering.html

http://www.techlicious.com/blog/gmail-spam-filter-artificial-neural-network/

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Thanks Eddy and Pondus!