The Abort method doesn't abort the operation immediately, but only if it doesn't finish within the time interval specified by Pop3 object's AbortTimeout method (which is 3000 by default - 3 seconds). Unfortunately, this information is missing from Abort method's documentation.
If you wish to abort an operation immediately, simply call Pop3 object's Dispose method. The Pop3 instance won't be usable any more after this, but that would be the case for Abort method as well if the time specified by AbortTimeout actually elapsed. (Reason: The POP3 protocol doesn't support aborting message retrieval.)