GMail woes: Error 550 5.7.1

I have a series of Inbound email servers and virtual domains that send out email via one smtp outbound server. We do absolutely no SPAMMING or Bulkmail of any kind. outbound server is Centos 7, postfix, dovecot (handles some inbound), sasl2 setup. Each of the servers has a DNS Record with an SPF allowing the IP4 of the outbound mail server to deliver mail on their behalf. v=spf1 mx a ip4:nn.mm.xxx.yyy -all On one of the inbound servers I have also added a _dmarc record to the DNS (trying to solve this gmail problem) v=DMARC1; p=none; Gmail keeps inconsistently bouncing the mail. I suspect some of their workers are configured differently to others. Also suspect this is a means to getting everyone on their domain (but that's for another day) I have the domain verified on postmaster.google.com but no actual stats show up (I'm guessing the traffic is too low) I get two messages typically Error 550, "5.7.1", Unauthenticated email is not accepted from this domain. & 550-5.7.1 [45.79.214.141 12] Our system has detected that this message is 550-5.7.1 likely unsolicited mail. To reduce the amount of spam sent to Gmail, 550-5.7.1 this message has been blocked. but the message content is literaly just regular text email with no links. I am looking at implementing DKIM on the outbound server but given this is sending mails from various other domains wanted to understand how this configuration could be made to work. Questions. 1. Does DKIM need to be done differently for each mail domain ? 2. How does implementing DKIM affect Gmail and will it solve the bounce problem any research links, suggestions or explanations gratefully accepted.