Yup, OpenBSD + OpenSMTPD off a wholesaleinternet dedicated box, for roughly a dozen domains. Previously, Postfix and/or qmail.
If anyone's got any tips or contacts for getting off of Microsoft's (live.com, outlook.com, hotmail.com ,etc) blacklist, I'd be much obliged. Their webform for such matters goes nowhere, and postmaster@ goes unanswered. It apparently has to do with the ASN of my hosting provider, but I've been on the same IP for 6+ years now and have done nothing untoward with it.