The subject says it all really. Ah, the joy of software when it works and the sheer f***ing rage and frustration when it does not.
For some reason which is a complete and utter mystery to me but not, I hope, to someone here, Avast has started to interfere with Mozilla Thunderbird’s ability to get emails from my Hotmail account. Only when I turned off Avast’s Mail Shield did everything work sweetly as, until only recently, it used to do.
Any ideas?
Was something else going on when the changes happened? I mean, some program update of avast? Or some install of some other software? Windows Updates? Web Browser updates? Email account changes? Email SSL settings changes?
Check your incoming and outgoing security settings in thunderbird, are they still set too none ?
Avast is constantly updating itself. The problem with Thunderbird has been happening intermittently since December 28th (when I returned from holiday, so it may have started before then). Today I hit on the idea of turning off the Mail Shield and Hey Presto! Email joy again. I have not installed other software. As for Windows, the only updates I accept are the ones that it forces on me automatically (which is not to say that they are harmless!)
I shall repeat myself - are the security settings in Thunderbird still set to none ? as they should be if you want avast to work with thunderbird.
If your now disabling the avast mail shield then you would also need to re enable the security in thunderbird.
I use Thunderbird 9.01 with Avast IS 1367 with a Hotmail account and email certificate. craigb is right there because if you disable Mail Shield and you leave Thunderbird set to “none” you are vulnerable there. Often I set mine to “none” but when I encounter difficulties or I seem to see that the servers are slow, I use SSL/TLS for POP3/IMAP and STARTTLS for SMTP. But that’s rarely. Mostly it’s the servers of Microsoft that’s a bit slow on my connection.
Thank you for both your initial response and this repetition. I did not respond to your first reply because I had to go out in a hurry directly after having replied to ady4um. Otherwise you would have received the courtesy of an immediate response to your question, which, now that I have got back home, I will address forthwith.
To answer you and craigb (both posts): No, they are not set to “none”, they are set as SSL/TLS for POP3/IMAP and STARTTLS for SMTP. Which is where they have always been set as far as I know, both before I started using Avast! and after. There was no problem at first. However I will now experiment with setting them to “none” and re-enabling the Mail Shield.
Thank you both for your help.
Yes the Hotmail servers are fairly slow and the problem with connection is amplified even more so when connecting through Windows Live Essentials to Hotmail, thats why Thunderbird is the far better option for a client as it’s resource - bandwidth usage is very low.
Yep! set those settings to none for both SSL and STARTTLS and your emails will come in properly and be scanned by avast.
If there are old SSL connection settings in the Mail Shield\expert settings\SSL settings then you can delet them and they will be recreated when you send and recieve your next email ( send yourself an email ) 
Yes the Hotmail servers are fairly slow and the problem with connection is amplified even more so when connecting through Windows Live Essentials to Hotmail.......If you use Windows Live Mail - client, then you dont have to sett up any SSL......you use Deltasync.....and IMO the best program for ms mail accounts you get full sync, including contacts
I aggree that the Live Mail client is the better program for hotmail but there server connections always timeout for me which is why i always suggest Thunderbird, or even better is to connect straight to Hotmail direct - no client.
If I set the security settings to “none” then I am informed that my password will be “insecurely transmitted”. I am not keen on the sound of that. If I choose the option “encrypt password” Hotmail is not keen on that. What should I do?
for avast to be able to scan SSL mail, you must dissable the SSL settings in your mail client and let avast do the SSL connection
avast! 5.x: Some e-mails are not scanned by the Mail Shield
http://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=458
OBS: if you dont do it, you are still protected. Microsoft scan all msn / Hotmail / Live mail accounts with Trend Micro AV
Ok. Thanks Pondus. 
I too have experienced issues with Avast 7.0.1474 preventing Thunderbird 17.0 from downloading email messages from an IMAP server. I get my email just fine when using my Comcast Cable modem internet connections but continually get timeouts from Thunderbird when running on my parent’s 1.5 Mbit ADSL modem. I first changed the timeout settings in Thunderbird but to no avail. Then I opened up all the ports on the router and that didn’t help either.
The solution was to turn off the Mail Shield in Avast for 10 minutes. Thunderbird downloaded all my emails just fine while Avast was disabled. Interestingly, when the 10 minutes are over and the Mail Shield is reactivated, I continue to get my emails. So somewhere a sticky flag got set in Avast or Thunderbird.
I do have a ton of emails in my inbox ( > 31,000). The timeout occurs while “Opening folder INBOX….”. Thunderbird memory footprint is 131 MB.
Details:
Intel Core i7-3610QM CPU 2.3 GHz with 8 GB RAM
CPU usage: 10% Memory Usage 4.37 GB
Windows 7 64 bit
Avast Free Version 7.0.1474
Thunderbird Memory footprint from Task Manager 131 MB
Unread: 3224 (messages)
Total: 31698 (messages)
mail.server.server1.timeout user set integer 29
mailnews.tcptimeout user set integer 200
network.http.connection-timeout user set integer 300