How can I verify my configuration doesn't have errors?
$ milter-greylist -c config file "/etc/mail/greylist.conf" is okay
If you get messages like these in your log when you start milter-greylist it's because the parser that checks the database at startup is more strict than the one that is used to insert new entries. The warnings are harmless.
Mar 16 20:48:39 mailsrv milter-greylist: unknown token "," line 998, skipping Mar 16 20:48:39 mailsrv milter-greylist: unknown token "@" line 1267, skipping
If you get log lines like these you can try increasing the timeout values for the milter, see Configuring Sendmail for milter-greylist.
Dec 8 07:03:26 debian sm-mta: kB7K3A2j012720: Milter (greylist): timeout before data read
It could also be milter-greylist dumping its database to disk. You can control the database size by adjusting the time to keep greylist and autowhitelist tuples.