in the command line option, you can use -k. here is from fetchmail's man page:
...
-k, --keep
(Keyword: keep) Keep retrieved messages on the
remote mailserver. Normally, messages are deleted
from the folder on the mailserver after they have
been retrieved. Specifying the keep option causes
retrieved messages to remain in your folder on the
mailserver. This option does not work with ETRN or
ODMR.
...
and in the config file, fetchmailrc, you can use keep option. just like this:
poll ...
user xxx with password yyy is zzz here
options keep
Best regards!