======== Required information ====
- iRedMail version (check /etc/iredmail-release):  0.9.5-1
- Linux/BSD distribution name and version:  Ubuntu 16.04
- Store mail accounts in which backend (LDAP/MySQL/PGSQL):  Mysql
- Web server (Apache or Nginx): nginx
- Manage mail accounts with iRedAdmin-Pro?  Nope
- Related log if you're reporting an issue:
====

I use an alias on my mail server to forward to a small list of people (less than 20). 

Intermittently I'll get this message from Google for the gmail address on the list:

"Our system has detected that 550-5.7.1 this message is likely unsolicited mail. To reduce the amount of spam 550-5.7.1 sent to Gmail, this message has been blocked. Please visit 550 5.7.1  https://support.google.com/mail/answer/188131 for more information."

But it's only intermittent, which strikes me as weird.  Doesn't seem to be related to who's sending, or what domain they're on.  Some get through, some don't.

My domain is verified in Google Postmaster Tools and does not appear as blacklisted in DNSBL.  I have valid SPF and DKIM records.

Any idea what might be happening?