Addressing a faculty member I interviewed I know there are other questions about how to address faculty members, but this situation is a bit unusual, so hear me out:

I'm in my last year at an undergraduate institution. My department head is very conscientious about using titles, but has for some time now used his first name and the first names of other faculty members when speaking and corresponding with me (not something he usually does). He recently asked me to conduct interviews with candidates for a new position in our department, and introduced all the candidates to me using first names. We've now hired one of these candidates, and I need to get in touch with this person via email for the first time. 

How should I address the new hire? Using "Dr X" feels artificial given our prior interactions (plus, I need to mention this person's spouse, who's also Dr. X), but I don't want to assume a level of familiarity with which the new hire is not comfortable. Of course I can ask about it in the email, but I've got to get past the initial salutation and welcome-interlude first!
If the emails are for some kind of 'official' purpose, then there is no harm in mentioning "Dr.". The person, if he feels uncomfortable, then he would reply back with a note asking not to be formal. This has happened with me quite a few times.

If the email is not for official purpose but for starting a conversation in a written manner, then go with "Hey X" or something similar.

Anyway, I would not overthink on these aspects. I would always go with the safe path by mentioning "Dr.".