I have been using a ".capital" email domain for work for the last few years, and the only issue I've encountered is form validation regexes that believe that the TLD portion must be between 2 and 5 characters long (literally "{2,5}" every time---must be copy-pasted from some regex recipe). I feel like it might be becoming less of a problem, but still something to keep in mind.

I use Office365 to host email, and have had no issues whatever with delivery.