We have the same issue, Since we have lots of users forwarding to gmail and we have lots of 3rd application using our email system Google is blocking for a prior of time (usually 1-2 days) before letting emails go through.
Basically the first step is to check to see who is sending out bulk emails, you can see this in the dashboard or in the mysql throttle table.
Once you determined who it is you can take appropriate action.We are still looking for solution on our end.
1)We have reduced the amount of emails a user can send within 1 hr. This has helped allot.
2)Looking at implementing 2+ ips for MTA
http://www.iredmail.org/forum/topic5177 … y-mta.html
This might help with ip blocking.
3)Looking at using sendgrid or mailgun for some domains that are doing bulk.
4)Another possible solution would be to Que gmail.com and google apps domain and only send out a batch at a time.