I'm the sole IT admin for a small organization and we've recently started having issues with some our customers rejecting our email as spam, with the commonality being that all of the rejections are customers that are using Barracuda for their inbound filtering (verified via MX records). We have 12 customer domains that are now rejecting us (as reported by my users). We're 100% Office 365 hosted Exchange shop (no on-premises servers), and we also use Barracuda for our inbound filtering, but do not route our outbound email through Barracuda (our sending servers are Office 365 mail servers). The issue began occurring on June 13th for all the domains in question, which makes me wonder if it's possibly related to a policy change in the Barracuda filters. Nothing changed in our DNS or Office 365 setup before the rejections started occurring, and the only change I've made since is to add the required DNS records for DKIM and DMARC and enable DKIM in the Office 365 Exchange admin console (our domain had an SPF record configured correctly prior to the issue).
The non-deliverable message provides the following rejection code (the same for all of the rejections):
550 5.7.350 Remote server returned message detected as spam -> 550 permanent failure for one or more recipients
(I can post headers/further details if it would be helpful).
We're sending from the domain wwf-mgt.com.
I've spent the last 2 weeks setting up and/or validating SPF, DKIM, and DMARC for our sending domain, to no avail. I've used MX Toolbox to verify our SPF, DKIM, and DMARC setups and everything looks good. We're also not experiencing rejection from any other customer/vendor domains.
I guess my question is - has anyone else experienced similar rejections, specific to Barracuda, starting on or around June 13th? I've run out of ideas at this point, and I would be more than happy to try any suggestions that the community might have.
Thanks in advance to anyone who responds.