Thank you for the information, Tom.
Please open AVG Antivirus free program.
Click on "Menu->about" and check whether it shows 21.1 version.
If not, get into "Menu->Settings->Update" and perform the program update.
Once it is updated, restart the computer and check the status with email error with web shield turned on AVG Antivirus free.
Keep us apprised about the status.
Thank you 
Yes I use the free version.
I cannot attach the images. Whe I click on the mountain symbol everything is gray and inactive.
here the text:
"Sending of the message failed.
Peer’s certificate has an invalid signature.
The configuration related to mx.freenet.de must be corrected."
and there is another one (german) about the certificate.
how can I send you the screenshot?
No, this does not work either.
In an other thread seems to be be the solution -
AVG Community
I asked for the the details to this already in my first email to the support but still did not get the details how to do it.
No, this does not work either.
In an other thread seems to be be the solution -
AVG Community
I asked for the the details to this already in my first email to the support but still did not get the details how to do it.
Several times I got the problem of not being able to send or receive emails through Thunderbird . (but there have also been phases without the problem)
When I tried to send emails I get the following message:
============================
Sending of the message failed.
Peer’s certificate has an invalid signature.
The configuration related to mx.freenet.de must be corrected.
==============================
When I disable the Email Shield, then all was OK…I could send and receive emails.
With the email shield enabled I cannot send or receive (same error message).
Thunderbird 78.7.1 (32-Bit) (but I had the problem also with other versions of thunderbird)
Would be grateful for help!
If I follow your Link I can`t find any help related to my problem.
None of the 4 topics in your search result has anything to do with invalid email certificates.
Or please let me know where to find this.
Please send me precise step by step instructions - Thank you!
Use the AVG Clear (https://www.avg.com/en-us/download-thank-you.php?product=REM) and restart the PC couple of times.
Confirm that you do not have any other security applications other than AVG.
Make sure you are able to send an recevie emails when there is no AVG in your PC.
Reinstall the application from the below link and let us know :
https://www.avg.com/en-us/free-antivirus-download
If the issue persists, we will provide addition support.
Thank you for the information.
In this case, we request you to reinstall the program and check for the issue again.
Keep us posted.
Hi Tom,
We are sorry to hear this.
We will help you to resolve the issue.
Please follow the steps in the article and check for the issue again https://support.avg.com/SupportArticleView?urlname=AVG-Troubleshoot-invalid-email-certificate
Keep us posted.
If I follow your Link I can`t find any help related to my problem.
None of the 4 topics in your search result has anything to do with invalid email certificates.
Or please let me know where to find this.
Please send me precise step by step instructions - Thank you!
Now I have this update:
21.1.3164 (Build 21.1.5968.630)
Same problems, unfortunately.
Hi Tom,
We are sorry to hear this.
We will help you to resolve the issue.
Please follow the steps in the article and check for the issue again https://support.avg.com/SupportArticleView?urlname=AVG-Troubleshoot-invalid-email-certificate
Keep us posted.
Now I have this update:
21.1.3164 (Build 21.1.5968.630)
Same problems, unfortunately.
Still the same error.