Thunderbird can't stop your provider from deleting them, if that is their policy.

But I must say I have some uncertainty myself in what happens if you have moved the message to another folder and then delete it.

Traditionally, POP accounts had small storage limits because they assumed you'd download everything and in doing so remove it from the server. If you elect to leave it on the server then it just accumulates and eventually might consume a limited server side quota. So you have to manually check the server (somehow) and manage the amount of material left there.

All of this is why I use IMAP. I can see what is and what isn't on the server and I avoid these uncertainties.

I simply want to know what happens when I check the option in Thunderbird Leave emails on server until I delete them, what is cause and affect when the option is selected, simple question or not ?

Server Settings option: Leave messages on server This means when you have a pop mail account and you download emails from the server, an instruction is sent to the server to keep a copy of that email on the server. Usually, by default, the server will auto delete any downloaded emails as explained by Zenos. So it means exactly what it says - the message is left on the server - it is not auto deleted upon download.

If you also select option: Until I delete them. When you delete an email in Thunderbird and compact folders to remove all traces of hidden marked as deleted emails then the email is also deleted off the server.

However, I always find that any downloaded email I auto filter directly to the 'Deleted' folder, does not get deleted off the server when I empty 'Deleted' folder. So, I've also added an extra action in the Message Filter to 'delete from pop server' and hopefully this will sort out that issue.

This could be specific to the server - BT, so I have to logon to webmail every so often to properly delete those emails off the server.

Thank you so much for the great information