An update on the ticket I opened with DH about this. And in response to lrosenstein's comments above, I certainly don't think it's necessarily DH's problem. Just that there's something going on between DH & Hotmail that causes sporadic bounces from HM to DH. I haven't been able to find any evidence it's happening from HM to other services (through my unscientific research). And like him/her, my problem also started in November.
I've exchanged a couple of emails with DH support (and one to HM, which has been acknowledged but not addressed). No solutions yet, but because of their comments I quoted on Jan. 15 in this thread, I thought I'd look at a lot of my received mail headers to see if I could see something.
Thinking that perhaps all the HM emails go straight to one server (if that's how it would work if they sent directly to an A record), I compared HM to other senders, and found no difference. Everyone went to both randymail-mx1.g.dreamhost.com and mx2, before and after November.
I did notice one clear difference between emails from Hotmail and all other emails though. Because my main DH email is set up as a mailbox at DH, and forwards to a gmail account, when I checked the headers of the forwarded mail on gmail, I noticed they also appended an SPF entry.
Every single email I checked that came from Hotmail (and I checked a lot) had
Received-SPF: softfail (google.com: domain of transitioning XXX@hotmail.com does not designate 208.97.132.xx as permitted sender) where xx wa variable, but always a DH IP.
Almost every other email had status "neutral" rather than softfail. A couple had "pass"
I have no idea if this could be related. It's past my understanding of email protocol, but from what little I know of SPF and its purpose, I wonder if it's connected. I gather "softfail" should never be a fail, but that sometimes hosts DO fail a delivery.
The response I got today from DH when I passed this info on was:
"It actually sounds like this data could be very useful in helping us resolve the issue. I'll pass this along to team responsible for resolving this issue, and hopefully we'll have a solution shortly."
Now I wonder, does the HM --> DH problem occur if the DH email then forwards the email? I have tried testing periodically over the last 24 hours sending to DH addresses that do forward, and one that doesn't. Unfortunately, I have had NO bounces.
A question for others who get this "relay access denied" bounce on mail coming to them from Hotmail: does your DH email address forward?