Hello Michael. I understand how something like this can really try your patience. I have sent article to your email id. Please run the tool and submit it so that we can analyze your issue to get is sorted. I really appreciate your patience. Please feel free to contact us back for any further assistance. Thank you.
Hi Aravind
Yes certainly frustrating. 
AT first glance, this looks like a Mozilla Thunderbird bug, but I now think that is less likely. I believe it is probably an AVG bug.
A bit more detail.
The Thunderbird mail client splits email into various "mailboxes". The contents of each mailbox is saved in a sequential text file (no extension) with an external index file with a ".msf" extension.
The "deleted" folder on my system is saved in C:\Users\Michael\AppData\Roaming\Thunderbird\Profiles\49js1hiw.default\Mail\Local Folders\Trash.
At irregular intervals Thunderbird loses the ability to delete emails. The cause allears to be an inability to write to the "Trash" file. Restarting Thunderbird does not help. The file cannot be moved, renamed or deleted using Windows Explorer, teporting that a program has the file open. After a system reboot, the file can be deleted.
I have discovered that by temporarily disabling AVG, I am able to delete the file without a reboot.
My diagnosis is that AVG may be holding the file open. It might be a leaked handle or possibly a locked thread caused by a race condition.
Because I get a large number of SPAM emails which Thunderbird auto-filters into the Deleted folder, that means that the Deleted folder gets much more use than any other, except the Inbox folder.
I am aware that AVG scans the mailboxes.
While I cannot prove that these two facts lead to my conclusion, I suggest it is worth investigating.
Regards
Michael
PS: I uploaded your diagnostics (with some misgivings due to the secutity implications) and got back a "request number" of 02245096
I think I have found a bug in AVG 2015 Free Edition on Win 7, 32 Bit.
AVG appears to be locking the file containing the 'Deleted' folder for my mail client, Thunderbird 38.1.0.
The file is
…\AppData\Roaming\Thunderbird\Profiles[my profile]\Mail\Local Folders\Trash
The problem occurs intermittently. Thunderbird loses the ability to delete messages and warns about being unable to write to its mailbox file. I cannot delete the Trash file in Windows Explorer (after closing Thunderbird). Clicking on "Temporarily disable AVG protection" in the AVG tray icon menu seems to release the lock and I can then delete the 'Trash' file. After that, Thunderbird then works as expected.
Any suggestions?
Hello Michael. Thank you for providing the case number. I have checked the reference number and found that you have successfully submitted the diagnostic information to AVG. Our level two technical team will analyze this and further communications will be done only via email.
Hello Michael. I would request you to click on Reply button and send the information without changing the original subject. Thank you.
Hi Aravind
Yes certainly frustrating.
AT first glance, this looks like a Mozilla Thunderbird bug, but I now think that is less likely. I believe it is probably an AVG bug.
A bit more detail.
The Thunderbird mail client splits email into various "mailboxes". The contents of each mailbox is saved in a sequential text file (no extension) with an external index file with a ".msf" extension.
The "deleted" folder on my system is saved in C:\Users\Michael\AppData\Roaming\Thunderbird\Profiles\49js1hiw.default\Mail\Local Folders\Trash.
At irregular intervals Thunderbird loses the ability to delete emails. The cause allears to be an inability to write to the "Trash" file. Restarting Thunderbird does not help. The file cannot be moved, renamed or deleted using Windows Explorer, teporting that a program has the file open. After a system reboot, the file can be deleted.
I have discovered that by temporarily disabling AVG, I am able to delete the file without a reboot.
My diagnosis is that AVG may be holding the file open. It might be a leaked handle or possibly a locked thread caused by a race condition.
Because I get a large number of SPAM emails which Thunderbird auto-filters into the Deleted folder, that means that the Deleted folder gets much more use than any other, except the Inbox folder.
I am aware that AVG scans the mailboxes.
While I cannot prove that these two facts lead to my conclusion, I suggest it is worth investigating.
Regards
Michael
PS: I uploaded your diagnostics (with some misgivings due to the secutity implications) and got back a "request number" of 02245096
Hi Balasubramanian
Your email said that further correspondence would take plce via email, but it didn't contain a "reply" address. I have some further information so I am posting it here.
Large amounts of SPAM email get filtered to the "deleted" mailbox (and thus the "trash" file. Many of these have viruses attached and AGV regularly detects them. This morning AVG detected a virus and reported it had been healed, while the trash file became locked at approximately the same time.
It looks like possibly AVG is trying to remove the file and failing (possibly a conflict with Thunderbird) and AVG is leaving the file locked. I don't think Thunderbird is ho;domg the lock as exiting Thunderbird does not fix the problem but temporarily suspending AVG does seem to release the file.
For completeness, the virus was Crypt4.BQMA, though I suspect that isn't important.
Hello Michael. What is Avinash requesting you to do is to click on Reply on the email from our level two technical team and not to delete the content of the email or the subject of it and please type in your reply and then just click send.
Hello Michael. I understand how something like this can really try your patience. I have sent article to your email id. Please run the tool and submit it so that we can analyze your issue to get is sorted. I really appreciate your patience. Please feel free to contact us back for any further assistance. Thank you.