This being an IMAP account, you can safely delete/replace the INBOX mbox file as you wish. This doesn't affect the messages in your Gmail mailbox. Think of the mbox as an offline local cache for the messages on the server. What you definitely should do, is add Thunderbird's Profiles folder, C:\Users\<username>\AppData\Roaming\Thunderbird\Profiles, to Bitdefender's whitelist (exclusions) list, or your specific profile folder so that it doesn't interfere with Tbird's normal filesystem operations. Antivirus programs are known to cause problems in Tbird, so you may also need to disable scanning of mail traffic if you encounter issues when sending/receiving messages in Tbird.

1. Yes, you can restore the quarantined files. You could choose the biggest of the two or the newest. It doesn't really matter, because Tbird will just create a new one if it can't use any of the restored ones for any reason. 2. You can delete and replace it with the restored one. They all contain the same messages, which remain untouched on the server regardless of what you do to the mbox files on your PC.

Thank you for your prompt response.

1. There are two files with the same name. One with .msf extension and the other without any extension. I understand that an INBOX mbox file means the INBOX file which has no any extension. Even after the quarantine I can see all the messages but could not open some of them offline. This is because they are indexed, and I can see them as far as the .msf files are there. If I go online and click on them, they will be downloaded again from the server. So whatever I do with an INBOX mbox file or INBOX .msf file, it will not affect the message in Gmail server, as far as it is an IMAP account, Right?

2. If I add the TBird's profile folder to Bitdefender exclusion list, how Can I protect it from Viruses? (I think, as far as it is an IMAP account, No need to worry about virus infection, Am I correct?)

3. There is a difference between the total number of messages in TBird's Inbox (44982) and Gmail Inbox (42011). How could the Tbird's number be higher than Gmail's number?

Thank you for your prompt response. 1. ...So whatever I do with an INBOX mbox file or INBOX .msf file, it will not affect the message in Gmail server, as far as it is an IMAP account, Right?

That's right.

2. If I add the TBird's profile folder to Bitdefender exclusion list, how Can I protect it from Viruses? (I think, as far as it is an IMAP account, No need to worry about virus infection, Am I correct?)

By whitelisting the profile folder, you're simply telling the antivirus' real-time filesystem shield to ignore the files used by Tbird for its normal functioning and to also exclude it from background idle scanning. If you download an infected attachment, it will not be saved in Tbird's profile folder. Instead, it will be saved in your Downloads folder, which is NOT whitelisted in your AV product, so the AV's real-time protection will still catch it before it gets a chance to run. In the off-chance that the virus actually manages to load into memory and run, the AV will still detect it in memory and terminate it. If you open an attachment directly in Tbird without saving it first, it will be temporarily saved in the TEMP folder, which also isn't whitelisted, so an infection will still be detected as soon as an attempt to save it to disk or access it is made. This has nothing to do with IMAP though. IMAP doesn't shield you from viruses.

3. There is a difference between the total number of messages in TBird's Inbox (44982) and Gmail Inbox (42011). How could the Tbird's number be higher than Gmail's number?

This is normal. By default, Gmail groups messages into conversations (threads), so the number of threads is smaller than the actual number of individual messages. You can change this conversation view setting in Gmail so that individual messages are listed instead. This will affect the reported total number in Gmail.

Thank you for your clarification.

By whitelisting the profile folder, you're simply telling the antivirus' real-time filesystem shield to ignore the files used by Tbird for its normal functioning and to also exclude it from background idle scanning. If you download an infected attachment, it will not be saved in Tbird's profile folder. Instead, it will be saved in your Downloads folder, which is NOT whitelisted in your AV product, so the AV's real-time protection will still catch it before it gets a chance to run. In the off-chance that the virus actually manages to load into memory and run, the AV will still detect it in memory and terminate it. If you open an attachment directly in Tbird without saving it first, it will be temporarily saved in the TEMP folder, which also isn't whitelisted, so an infection will still be detected as soon as an attempt to save it to disk or access it is made. This has nothing to do with IMAP though. IMAP doesn't shield you from viruses.

Actually I too thought the same thing. I should use INBOX mbox file instead of IMAP account as below. "I think, as far as it is an INBOX mbox file, No need to worry about virus infection (as it is like an archive file, no virus will spread)".

1. Yes, you can restore the quarantined files. You could choose the biggest of the two or the newest. It doesn't really matter, because Tbird will just create a new one if it can't use any of the restored ones for any reason.

All my messages are safe in Gmail server. But I could not work offline, even if I restore the quarantined files. 30000+ messages (totally 8 GB) were already downloaded before quarantine happened.

I tried deleting the INBOX.msf file and repair folder option. As you mentioned It creates new one and starts downloading from the beginning. It takes long time to completely download all the messages.

Is there any way to restore the already downloaded messages without downloading them again?

Is there any way to restore the already downloaded messages without downloading them again?

None that I know of. Normally, you would simply delete the msf file then restore the mbox file and launch Tbird, which you've already tried, but who knows what the AV may have done to the mbox file rendering it unusable by Tbird? The AV may have tried to quarantine the individual message that contains a virus, thus altering the mbox file or trying to, while Tbird also has its handles locked on the file because it is actively writing to it as it syncs the messages. That's an easy way of wrecking the mbox file. If I were you, I'd let Tbird create a new mbox file, but limit how many messages get downloaded. By default, all messages are synched regardless of age, but you can change that to say, six months old and newer, to keep the mbox file and your Tbird profile folder smaller for quicker backups. There is really no need to have ALL the messages downloaded for offline access IF you're always working online anyway. Also, Google recommends that you sync fewer messages to avoid exhausting the bandwidth limits imposed on Gmail IMAP/POP access, which would affect fetching of new messages once you've reached the limit.

None that I know of. Normally, you would simply delete the msf file then restore the mbox file and launch Tbird, which you've already tried, but who knows what the AV may have done to the mbox file rendering it unusable by Tbird? The AV may have tried to quarantine the individual message that contains a virus, thus altering the mbox file or trying to, while Tbird also has its handles locked on the file because it is actively writing to it as it syncs the messages. That's an easy way of wrecking the mbox file. If I were you, I'd let Tbird create a new mbox file, but limit how many messages get downloaded. By default, all messages are synched regardless of age, but you can change that to say, six months old and newer, to keep the mbox file and your Tbird profile folder smaller for quicker backups. There is really no need to have ALL the messages downloaded for offline access IF you're always working online anyway. Also, Google recommends that you sync fewer messages to avoid exhausting the bandwidth limits imposed on Gmail IMAP/POP access, which would affect fetching of new messages once you've reached the limit.

Yes, You are right. Downloading the last 6 months messages is enough to work offline. But, Why I am trying to download all the messages is the unreliability of Google services nowadays. You may know that, a few weeks ago, some Google services were affected, That time My account was too affected. I was unable to upload / download attachments in Gmail and unable to use Google Drive. This is not the first time. Google services were affected many times in the past. I think in 2011, some users' inbox became empty (however they restored the mails, later). So I would like to safely backup everything.

One more query. During the restore process, I came to know that we can read the messages by opening the mbox file in a text editor. If it is encrypted, we can avoid AV disturbing it. Is there any option available in Tbird to encrypt it? I found end-to-end encryption, but could not find anything for downloaded messages.

Why I am trying to download all the messages is the unreliability of Google services nowadays...So I would like to safely backup everything.

I am aware of the recent temporary outage of Google services. In that case, let it download all messages unhindered by your AV. Be advised that you should not treat the mbox file of IMAP-accessed accounts as a reliable local backup, but instead, as a temporary cache that get's cleared and recreated if it becomes obsolete/corrupt or unusable for some reason, as was the case when you couldn't force Tbird to re-use the quarantined ones.

For a reliable local backup, use local folders (under the Local Folders special account) and copy your messages to those folders.

You could also, alternatively, use a dedicated email archiving solution like MailStore Home to create a local backup of your Gmail mailbox.

Another option which I highly recommend for Gmail, is to use the Google takeout facility to download your entire mailbox as an archive containing a set of mbox files that you can directly copy to the Mail > Local Folders subfolder of your Tbird profile folder. The takeout facility does not suffer the bandwidth limitations that Google imposes on IMAP/POP access of your mailbox, so you're assured of an uninterrupted one-time download of your entire mailbox once the takeout is ready. This, once accomplished, will significantly reduce the number of messages and folders that you would need to manually create and store under the Local Folders account, because you would only need to copy new messages that arrived AFTER the takeout was processed.

If it is encrypted, we can avoid AV disturbing it. Is there any option available in Tbird to encrypt it?

No, there isn't. Whitelisting the profile folder is still the best option nonetheless, and achieves the same

Thank you for reminding me again (and again) that IMAP-accessed accounts are a temporary cache. For backup, using the Google takeout facility is a really good idea. Thanks a lot.

If Mbox file encryption is not possible, I would like to try Maildir, as if any virus found by AV, it can remove only that particular message instead of the entire folder. We can also avoid compacting (it sometimes shows error in Mbox) and Backups are easier and shorter.

I am aware that there are still many bugs in Maildir. Before going into it, I need your suggestion. Mainly I would like to know that Maildir-IMAP accessed accounts are also a temporary cache, like Mbox-IMAP accessed accounts? If anything goes wrong with Maildir-IMAP (EML files), then will all messages be still safe on the Gmail server?

It seems you're still not convinced about excluding Tbird's profile from your AV's activities, so I'm just going to point you to http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Anti-virus_Software and https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues then you can make your own mind about how to proceed further as regards your AV and Tbird.

All I can say about Maildir is try and see if it works for you. I never bothered with Maildir even though I see why it's a desirable storage scheme for some users.

Mainly I would like to know that like Mbox-IMAP accessed accounts, Maildir-IMAP accessed accounts are also a temporary cache? If anything goes wrong with Maildir files, then will all messages be still safe on the Gmail server?

I believe that to be the case. The server copies of your messages should remain intact on the server until an IMAP command is sent by Tbird (the client) to the server to perform certain actions on those messages. Sudden loss of your local copy of the message as a result of filesystem operations (e.g quarantine or deletion of individual email files) on your PC should never result in TB sending an IMAP command to the server to sync/mirror local actions that are external to TB. You can test this out, though, by setting Tbird to sync just a few messages (a few days worth) then delete them and see if the server copies are affected. Best to use test messages that you've sent to yourself.

No, I have no hesitation in excluding, But apart from this there are many other benefits in Maildir and I am getting errors every time when compacting is running in the background. I am getting an error for each folder. I have to click OK several times during the compacting process. That is annoying. Please take a look at the attachment.

You have provided me with a lot of information so far. I have learned a lot from you. Thank you so much for your valuable support.

I recommend turning off automatic compacting and only do it on occassion after taking Tbird offline and after a sizeable number of messages have been moved in and out of folders to justify the gains of compacting.

I recommend turning off automatic compacting and only do it on occassion after taking Tbird offline and after a sizeable number of messages have been moved in and out of folders to justify the gains of compacting.

As I have more than 100 folders, first I tried to compact through File>Compact Folders option. But if Tbird is offline, compact option is not available (Grayed out) under the file menu. Then I tried right clicking on a folder. It is not available there too. If Tbird is online then both are available. Can you specify? (Please see the attachment files.)

You're right. The Compact option is inactive for IMAP or online folders when Tbird is offline. It should, however, be available for offline (local) folders.

You're right. The Compact option is inactive for IMAP or online folders when Tbird is offline. It should, however, be available for offline (local) folders.

So, No way to get rid of the errors. 

I have decided to maintain two different profiles. One for Mbox and the other for Maildir. I am going to use Mbox for my primary email ID and Maildir for the secondary (unimportant) ID. So, I can choose a better one for me after a few months.