I'm using the recommended Fastmail setup with my own name server, which doesn't support DMARC out of the box. Is this something I should look into to improve deliverability?
I haven't bothered with anything beyond the recommended SPF and DKIM records, and am not aware of any issues. IMO it will be years before not having DMARC is a serious issue for delivery, and I expect Fastmail to tell me when that time is getting close and/or take care of it for me.