Quote:The PTR record for this IP points to mail.z3bra.org. I tried adding another PTR record for the virtual domain, but it's still propagating and I'm not sure an IP can have multiple PTR records.

Technically you can, practically this wouldn't be a good idea, because it would work as round-robin, giving a random PTR-record back every time. Does your virtual domain have an A-record? If not: Set it. That could be it.

Quote:I'll setup DKIM/DMARC then, as it's the last point I can improve.

Setting DKIM up isn't a bad idea, but especially for DMARC I'd recommend thoroughly reading about it beforehand. Because it's not a simple solution to problems and comes with a whole set of its own.

Quote:As far as I know Microsoft mail servers (hotmail, live, etc..), won't accept any email, complete nuke them, if they aren't signed by the mail provider.
Gmail is a bit more loose and will mark it as spam.

You're misinformed here.