Emails not deleting off the server which is filling up?

My work email is filling up and is not deleting off the server once the emails are downloaded to Mail. In accounts, advance, the box is ticked saying 'Remove copy from server after retreiving a message: after one week'. In the last week I probably sent and recieved about max 100 emails, probably more like 60. The account type is POP and I am downloading messages from a server (?) that is linked with Squirrel Mail.

In advacned if i hit the box 'Remove now' will it remove all the messages from the server and not from my computer?

Thanks!