Google detects me as spam

I've been asked to look at some trouble I've been getting with our payrol system which is sending emails to our users. My problem is any email going through google (virgin.co.uk, gmail.com etc) fails saying 'Our system has detected an unusual rate of 421-4.7.0 unsolicited mail'

The precise contents is (I've replaced the users email with USERSEMAIL, and our MYEXTERNALIP):-

Mar 18 08:25:49 ubuntusquid postfix/smtp[16132]: C86177A0AF8: to=<USERSEMAIL@virginmedia.com>, relay=alt2.aspmx.l.google.com[74.125.130.26]:25, delay=433182, delays=433176/0.04/3.5/2.3, dsn=4.7.0, status=deferred (host alt2.aspmx.l.google.com[74.125.130.26] said: 421-4.7.0 [MYEXTERNALIP      15] Our system has detected an unusual rate of 421-4.7.0 unsolicited mail originating from your IP address. To protect our 421-4.7.0 users from spam, mail sent from your IP address has been temporarily 421-4.7.0 rate limited. Please visit 421-4.7.0 http://www.google.com/mail/help/bulk_mail.html to review our Bulk 421 4.7.0 Email Senders Guidelines. fs8si24099257pdb.76 - gsmtp (in reply to end of DATA command))

I have gone through the link http://www.google.com/mail/help/bulk_mail.html and done everything I can, but still getting the issue. Ive gone through forums, and done everything I can think of apart from things lile "dont send same subject" or "dont send encrypted attachments" but obviously I cant get around this with our payrol server.

Emails from the printers all seem fine to the users personal email address, so its only effecting the payrol emails, and they constantly fail even after several days of retrying.

Im using postfix on ubuntu server. Reverse DNS is setup correctly, and my IP and domains are not blocked on any spam lists.

Does anyone has any ideas how I can fix this going forward apart from us refusing to use users personal emails to google emails.
tonelm54Asked:
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ChrisCommented:
Is the external IP being used by your payroll system also used by network clients? It's possible that there's an infected PC or server on your network sending out spam. My first step would be to block port 25 outbound on the firewall for all devices except those that you are sure are trusted, this will prevent any outbound mail from potentially infected clients and could, hopefully, resolve your issue.

Alternatively, if you have a bank of external IPs, ensure that your payroll system is assigned an address that isn't used by any other devices.
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