How do POP3 clients delete emails later?

Question

For example Thunderbird has the POP3 option to "Leave messages on the server." If this is checked then you can check "For at most __ days." How does it delete your emails later? Does it tell the server at the time of downloading to wait however many days then delete the downloaded emails, or does it keep track of time itself and when receiving emails in the future delete them then?

Also, what would happen if you had it set to delayed delete your emails and then later set it to leave them. Would the ones that had been downloaded during the first period but hadn't been deleted yet be left alone or deleted like originally planned? I'm also wondering about the reverse of this. If you have your program leave your emails then switch to have it delete them, would all of them get deleted?

Do all POP3 clients treat these situations the same? I'm wondering specifically about Thunderbird and Windows Live Mail.

Answer 1

The answer to your first question is that the client deletes the messages (the POP3 protocol doesn't have provisions for delayed deletion)

As far as delayed deletion and then changing the setting, it depends on the client, but I would suspect that if you changed from 10 days to 30 days, the emails that were 11 days old or older at the time of the change would be safe until they were 30 days old.

If you were to do the inverse (30 - 10), I would expect the emails 11 days old and older to be deleted from the server at the next update.

For how individual clients behave, I suggest testing; It's the only way to be sure.