Where/How does fetchmail store which mail it has seen?

Question

I use fetchmail to fetch mail from a remote POP3 server and deliver it to my local mail server. Now I want to migrate fetchmail from one server to another. But where/how does fetchmail store/know which mails it has already seen? I don't want to end up with duplicates in my inbox after migration. My fetchmail configuration does not delete seen mail, so there are still mails available on the remote server that were already processed by fetchmail on the old server instance.

Answer 1

fetchmail by default stores ids of fetched emails in .fetchids file (~/.fetchids).
See --idfile in man fetchmail