Monitoring postfix mail throughput

Put messages in a HOLD state

/etc/postfix/main.cf:

smtpd_recipient_restrictions = 
    ...
    check_recipient_access hash:/etc/postfix/hold

/etc/postfix/hold:

gmail.com        HOLD
blah.com         HOLD

Make sure you run postmap hash:/etc/postfix/hold whenever you update the file.

If you want to release all messages on hold, use postsuper:

# postsuper -H ALL

I found out the reason (with help of the kind guys of the opendkim mailing list):

In my opendkim.conf file I had only the field

AlwaysSignHeaders       From,Sender,To,CC,Subject,Message-Id,Date

specified. It turned out that you need to add the SignHeaders field too, so now the config looks like:

...
SignHeaders             From,Sender,To,CC,Subject,Message-Id,Date
AlwaysSignHeaders       From,Sender,To,CC,Subject,Message-Id,Date
...

that's it.

Edit:

The folks from opendkim noted that AlwaysSignHeaders is deprecated, OversignHeaders should be used instead.