[brookside]

There are many possibilities around the candidate's behavior. They could be going through a manic episode, or autistic, or just a jerk, or scammer.

OP handled it well, but two things I would do differently in responding.

1. "Sorry you feel that way". I never apologize for other people's feelings, only my own actions, when am am sincerely sorry. "Not-really-apologies" are, IMO, always in bad taste.

2. Not sign emails using "best". Best what? Obviously this is up for interpretation but a dangling best is (IMO) corny and exudes "I am writing unnaturally and I think this is how professional people write"

[romanhn]

Best is short for "best regards" and is, in fact, how many professional people write. I agree with the rest.

[ryandrake]

I wouldn't have even responded to the first E-mail. The guy is clearly trolling at that point, and nothing good can come from feeding him.

[LoganDark]

Not sure if being autistic on its own would cause this, but I suppose it's often comorbid with other things. Otherwise I agree.

[pavel_lishin]

I would have not responded at all.

[brookside]

Totally valid, and agree.