Thunderbird stops downloading new emails

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Happy new Year! I'm back with more details. bob3160 thank you for your reply but I don't have any error messages in the logs about invalid certificates. Also I suppose the error would appear with every message I receive and not just emails with specific structure. The log file Mail.log show "--POP CATCH (...) in Retr" repeating each time thunderbird tries to download new email messages and fails.

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Any other ideas for things to try?

[Pondus]

Is this your Gmail account?

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No it isn't a gmail account. It is a personal email account I have as a teacher in greece from sch.gr.

[Pondus]

you may contact the mailprovider and check if they have spam/virus protection on the mail server

or you can test here
http://www.emailsecuritycheck.net/
http://www.aleph-tec.com/eicar/index.php

if they do a alternative is to turn off mail shield or try another mail client  http://www.opera.com/computer/mail