Page History

...

The --after=1072933200 option tells mass-check to ignore messages older than 18 months ago a certain cut-off point (in this case January 1 2004). This is useful if your corpus has older messages intermingled with your newer messages. This should be old enough to include plenty of good non-spam mail (which doesn't go out of date as quickly as spam) – we will be removing too-old spam in a separate step, anyway, so don't worry about spam.

If you have an unusual network layout, you may need to specify
trusted_networks and/or internal_networks in the
spamassassin/user_prefs file. But SA should be able to infer it in most cases. A good way to tell is if you see no SPF_PASS results – SPF will not be used if the message passes through one or more trusted relays.

...