Deleting emails (General questions)

Hi,

I want to download 'new' emails only and delete emails from the server based on a blacklist

Firstly whats the best way to make sure only new emails are downloaded. I assume this will use a list of UID's previously downloaded but does this mean I have to check every mail header on the server?

Also deleting emails, what if someone had deleted some emails before the prog deletes them. Does it screw up the message numbers. Again do you have to go through each email on the server to find 1 email to delete?

I'm sure someone has a better idea on this. Thanks