I managed to activate my device by capitalizing the domain in my email address (i.e. DOMAIN.COM instead of domain.com), and went back and read the email headers on the message I finally received (I used to run a regional ISP, including it's email server, back in the dial-up dark ages).
Roku is using Amazon's simple email service (Amazon SES). Looks like it's doing some filtering and restrictions on delivery (To: ) email addresses, deciding which domains they think are acceptable to send email to. In other words, inept filtering for probably spam prevention, but done wrong (you'd filter on the domains email originates from, which would be pretty bizarre when sending validation emails from already approved accounts). Not very good filtering, either, if changing capitalization defeats it - not that I'm complaining since it got me through the morass.
Judging from the responses of their tech support, along with a phone conversation I had with them, they don't have any idea how to troubleshoot this. They did spend a fair amount of time on the phone with me, can't fault their efforts, but it's an issue their developers need to resolve. Looks like SES provides tools for this, starting with this page in SES's online documentation.
Pure speculation on my part, but I'm guessing they outsourced their validation programming and those developers may have moved on to their next projects. Likely the Roku trying to be validated phones home to something running on AWS (or connects to an AWS app if you prefer), which then uses the SES API to send an email. Something inside SES (or maybe in Roku's own validator) decides that the destination email address of a previously verified account is unacceptable, and this attempt at validation is very broken, along with being inappropriate (validate destinations for valid accounts? Huh?) to begin with. If I can get this far with 10 minutes of looking at email headers and some web searching, and they haven't fixed it by now it's beyond their in-house skills. Guessing some chatting with SES's tech support could have sorted it out for them pretty quick.