Welcome and thank you for your question.
Thankfully you fully realize peeking at someone else's correspondence is a fundamental betrayal of trust so I won't berate you on that any further :)
I don't know about being ‘threatened' but I'd be mighty pissed at this woman contacting your boyfriend. There is something going on in her life right now and it certainly sounds like her happy marriage isn't quite so happy or she wouldn't be ferreting around contacting ex-boyfriends. Would she?
Can you confirm with me that he ‘joked' the pictures turned him on – was this in the recent email that you accidentally read in the last few days?
I'd like a bit more clarification as younger guys do sometimes think with their dicks which wiley females can take advantage of, sadly.