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Per-user greylisting
You can set it so that only particular users have their emails grey-listed.  e.g. in /etc/postfix/main.cf have:

smtpd_restriction_classes = greylist
greylist = check_policy_service inet:127.0.0.1:60000

And add the following to smtpd_recipient_restrictions in /etc/postfix/main.cf:

    smtpd_recipient_restrictions =
            [other options that normally go here]
            check_recipient_access hash:/etc/postfix/greylist_optin

(Note that you would have this set in smtpd_recipient_restrictions instead of the "check_policy_service unix:/var/spool/postfix/postgrey/socket" setting mentioned in the previous section.

In /etc/postfix/greylist_optin put users who should be greylisted followed by 'greylist' (the class defined above):

fred@example.com    greylist
mary@example.com     greylist

Sending mail to those users should now get greylisted (might need to 'postmap /etc/postfix/greylist_optin' before restarting postfix).

In /etc/postgrey/whitelist_clients you can add domains (domain names, one per line) that you think should not be greylisted.  e.g. domains of users that you deal with frequently (maybe avoiding common sources of spam though like yahoo.com, hotmail.com, etc).