Is this an IMAP account? Some IMAP servers just do not support those flags and they are lost on synchronization.

No, I am using Thunderbird to access my Verizon account which has a POP server.

Now what would be causing the MSF file for the folder to disappear or be corrupted. Is your anti virus scanning the folder? do you have some sort of disk space optimizer?

I have disabled scanning of my Mail folder and I do not use a disk optimizer on that drive. Any other ideas?

Thunderbird stores those flags in an MSF file.

So in your profile in the Comcast folder will be inbox and inbox.msf

the file with no extension is the mail. The MSF is basically an index into the mail, but it has some other uses and one of those is the reply flags. I suggest you watch that file and see if it disappears while Thunderbird is closed. It will be automatically regenerated if corrupt or deleted. but the reply flag data will be missing.

I asked about disk optimization programs as they sometime in the past have deleted MSF files in a futile grasp for more disk space. But any security program might look there, hence my suggestion to watch the file and see what is happening.

It might also be causes by a bad add-on. Do you have any? It would be painful to run Thunderbird in safe mode until the issue did or did not occur when the frequency is a mystery, but that is the normal way to exclude add-ons as a cause.

I'll keep an eye on the MSF file but I'm using Thunderbird throughout the day so it may be hard to catch.

I have the following add-ons:

• Theme Font & Size Changer 37.1
• Select Inbox 2.5
• InportExportTools 3.2.1