Having the same problem here to. We send mail from a gmail address, to a distribution address on exchange. All gmail targets in that distribution list most often end up deferred as rate limited unsolicited mail. Our DKIM, SPF, DMARC all check out. Once the mail arrives in the gmail inbox (sometimes it does after several hours). Looking at the mail headers, there is NOTHING in the mail headers, nor in gmail analysis of it that warnings that anything is weird with it.
This very much smells like a gmail issue.