POP3 server assigns a unique ID to each mail in the mailbox. This unique ID is the same each time you connect. This makes it possible for an application to keep track of the IDs of mails it retrieved (and that's what most POP3 client applications do)
The drawback of this is that some kind of persistent storage for the unique IDs is needed, so this is not an appropriate task for a POP3 library like Rebex POP3 for .NET. This means that you have to keep track of unique IDs yourself in your application.
To retrieve the list of mails including unique IDs, use GetMessageList method. For mor information about unique IDs and sequence numbers, check out the tutorial.