There is no reason for any change to the sieve code when you simply disable a POP link. The relevant code will simply never have any effect. Also, whether you keep or delete emails on the original server from which you fetch using a POP link is not controlled by the sieve code.
Are you seeing any unexpected results from use of these identities, or are you just trying to understand the sieve code to gain a better appreciation of how FastMail works?
|