Online help

it's best not to check this because you will want your mail to be deleted from your server when Ezian picks it up.

When you might leave mail on mail server
When you have two or more locations where you Get your mail, you might find leaving mail on server useful.  For example, you would want to check to leave mail on server if you picked up your mail from two different computers.  You would choose to leave mail on server on Ezian on one of the computers and not on Ezian on the other. This way when you used the computer where you wanted to keep and organize your mail, it would always remove it from the server. Leaving mail on your server is normally not a good idea unless you have a situation like that, and it is discouraged by the people who run the servers, because they don't want them to get overloaded with old mail.

Example of leaving mail on server
Some users have two computers, one at work and one at home,and they have a business identity or email address and personal ones.  When picking up mail at home they have their home copy of Ezian with the business identity selected to leave mail on server and when picking up mail at work they have their business copy of Ezian with their personal identities selected to leave mail on server.  This way they can read both kinds of mail at both locations, and merely delete when they happen to get extra copies, but it makes sure that copies of personal mail are always filed on the home computer and business mail on the business computer.