What to do if e-mails suddenly disappear

Q: When I opened my Mozilla Thunderbird e-mail program, I kept getting the message: "Do you want to compact all local and offline folders to save disk space? This will save about 229 megabytes."

I always said no because I didn't know what "compacting" did. But I must have accidentally accepted, because now some of my e-mail folders are empty. It appears that every e-mail from before 2016 has been deleted. I contacted my e-mail provider, the University of Minnesota (which uses Gmail), but I haven't gotten any useful information. How can I recover my lost e-mails and prevent any more from disappearing?

Kathy Knauth, Minneapolis

A: The problem is most likely caused by an incompatibility between Mozilla Thunderbird and your antivirus software.

Thunderbird periodically "compacts" stored e-mail. It erases e-mails that the user has chosen to delete and scrunches the remaining e-mails. This doesn't save a significant amount of hard disk space (229 megabytes is only a tiny fraction of your hard disk's capacity), but it simplifies Thunderbird's data-handling, which makes it run better.

This compacting process goes to the heart of the unusual way that Thunderbird stores e-mail: E-mail files are converted into plain text format and many are combined into a single text files that are called "inbox," "drafts," "sent" and so on. If you later delete some of those messages, Thunderbird "compacts" a text file by calling it up, then storing the messages to be kept in a new text file. It then deletes the old text file, and replaces it with the new, smaller one.

The Mozilla organization, which created Thunderbird, says compacting shouldn't cause you to lose any e-mails that you intended to keep. But the way your antivirus program interacts with Thunderbird could inadvertently delete a lot of e-mails. For example, an antivirus program that's not fully compatible with Thunderbird may scan the e-mail files, detect one e-mail that it believes is dangerous and delete the text file containing that e-mail — along with all the good e-mails contained in the same text file. Why would an antivirus program do that? Because it's meant to work with mail programs that give each e-mail its own file.

• You may be able to recover your lost e-mail if your antivirus program "quarantines" suspicious e-mails instead of deleting them. Quarantined files still exist on your PC. Read "Recovering a quarantined Inbox" (see tinyurl.com/bkway).

• You can change some settings on Thunderbird and your antivirus program. For example, you can set Thunderbird to download each e-mail to a separate file (see tinyurl.com/22zpzy). Or you can set your antivirus program not to scan Thunderbird e-mail (this is less dangerous than it sounds, because the university scans your e-mail for malware before you receive it). Read "Keeping your antivirus software from deleting your Inbox" (see tinyurl.com/bkway).

• Switch from Thunderbird to another e-mail program. (For alternatives, see tinyurl.com/y7m4t8ba). Or, log into Gmail via its website (gmail.com).

• Change to another antivirus program. (For a list of antivirus programs that do and don't work well with Thunderbird, see tinyurl.com/2cz3lth).