Thunderbird does not make that file as a part of normal operations. Do you have some form of an addon doing backup, or some external backup application?

Thanks for the response. I have no addons, apart from a couple of language dictionaries. I do have an external hard drive, but only use windows backup, and I also manually backup some files there.

I have been using Thunderbird for many years, getting on for 4 years on this computer. My email accounts have not changed in that time, and I regularly delete old mail from the IMAP servers.

This is something new, it seems TB is backing up all the mail I get, even though on a daily basis I delete 90% of it.

What is the date of that file?

2nd July 2025 I can't see what is in the zip file.

Thunderbird uses that file name.

Deleting a back-up file is safe.

Of course, you will want regular back-up copies stored somewhere other than your internal drive.

How big is your profile folder without that file?

Profile folder is 301Gb, the zip file is 298Gb which makes it 3Gb.

I will delete the zip file, but how did it appear and get to that size?

I do not know.