Thunderbird: Using POP, how does the "Leave message on server" work when unchecked?

Question

I installed Thunderbird Portable because I wanted to copy an email from one account to another. I accidentally selected POP3 instead of IMAP. By default, the settings are "Leave message on server for at most 14 days" and "Until I delete them".

Of course I don't want my emails to be automatically deleted in 14 days. So how does that work exactly? Are they marked for deletion or would it be Thunderbird that would delete them if it was opened?

If they're marked for deletion, is there anything I can do to unmark them?

Thank you!

Answer 1

I had to do a little research.

Thunderbird does not mark the email. Instead, it keeps track of when messages were downloaded. After ‘x’ days it will delete them from the server.

If you change the option, it will affect all past and future messages that are not already removed from the server.

Here’s one reference about your question: http://forums.mozillazine.org/viewtopic.php?f=39&t=2756387

Answer 2

It's a basic difference between behaviours on POP3 vs IMAP - one you can use to your advantage if you so wish.
I'm over-simplifying, but in essence…

IMAP servers preserve all emails, in perpetuity, & your mail client on any machine directly accesses that server content. Any mail deleted from one client will delete from the server & therefore 'vanish' from all clients,

POP servers, on the other hand, transfer all content to you. It is then your job to maintain archives on each local machine. Any mail deleted from any given client has no effect on mail downloaded to other clients.

The POP client itself tells the server when to delete older messages, after a set time - as you see from your Thunderbird prefs, this is a client decision. Many IMAP servers will observe correct POP3 behaviour if you access them as though they were POP3. [MS Outlook can do this, idk about others.]

Which method you choose depends entirely on whether you want constant sync between all clients or whether you want a 'master client' to do your archiving.

Personally, I like the POP3 structure, as it allows my main desktop to be the master archive for all my mails, going back 2 decades, whilst my phone remains mainly empty, except for the last few days. Anything I delete from the phone does not get deleted from the master.

In your particular case, if you switch to 'Until I delete them' then the client will no longer flag for deletion anything you downloaded 14 days ago. Because of the way POP3 works, anything already downloaded to that client more than 14 days ago will have already been deleted, but it will simply stop telling the server to delete newer messages.
If you want to return to full IMAP behaviour, you will have to tell the client to use IMAP protocol - currently, even without the deletion, the server will be acting like POP3, ie no sync between clients.