Submitted a couple of support requests with OnMail. Firstly, a query about their "security scan" as it seems to have an issue with my .blog email - I asked if they would consider a feature to trust certain senders so they don't get flagged as suspicious. Next, I wanted more clarification on exactly what happens (and any potential impact) of connecting an existing email address to OnMail. They're not very clear with how it works and the last thing I'd want to do is mess up my DNS etc.
I've performed some checks and don't see why OnMail would have an issue with my .blog emails. I've got SPF and DMARC in place so it's pretty reliable that emails from me are from me.