IMAP accounts, whether on Thunderbird or other clients, manage the messages on server. What you see on PC are downloaded copies for your viewing, replying or forwarding. However, if you delete a message, whether online or at your PC, the message is gone. POP accounts are different and act as you are wishing IMAP did. So, if you want to keep messages on PC, yet remove from server, just copy the message to local folders prior to deleting.

Thanks, but this negates the apparent meaning of the option offered under retention policy of [don't delete any messages] that is also offered for IMAP accounts ?

Surely the software should be amended to follow the logical interpretation of option [don't delete any messages] I dont see that this option,though offered for IMAP, has any functionality?

The issue is that you deleted the messages from server, not through thunderbird. If you want thunderbird to manage your messages, then delete with Thunderbird, not from server. After deleting from server and starting Thunderbird, it does a normal syncronization and removes the copy from PC.

As said i must delete server emails but keep local emails in place and don't know how to do this, your solution would delete both without first making a copy of them,

Is the meaning of option [don't delete any messages] [don't delete any messages from server when messages are deleted locally]

It would be good if a software developer could look at this?

NO. My solution is to copy to local folders prior to deleting from server. The assumption of any instructions within a software program, whether Thunderbird or others, is that the information applies only if the function is being done by that program. If you must delete from server and keep on PC, you might consider switching to a POP account.

It would be good if a software developer could look at this?