<human>: Summarize the given text in a few sentences.

My partner of nearly 5 years is being harassed via email by an ex-hookup of his (the last romantic involvement pre-dates our relationship). She emails him once or twice a week, basically begging for contact with him. He's tried to tell her to move on, to tell her that he's in a happy and stable relationship, to tell her to look on the bright side, etc (all via email). He keeps the emails very short and does not try to be unkind to her, but does try to be extremely neutral.

I'm not threatened by this (so this isn't one of THOSE kinds of posts), but feel bad for him. She's from a phase of his life he'd like to put in the past and he really doesn't want to have contact with her, but when he tries cutting off all contact (as in not replying) she gets extremely agitated (emailing multiple times per day, sounding very depressed/urgent/incoherent, guilt trips, etc). So he has resigned himself to emailing her just enough to keep her from going full-on, totally-batshit crazy. Seems unhealthy for him (and for her, though, really, I just care about him).

Any ideas for him? If it was me, I would just stop replying and have a "what she does is her problem, not mine" attitude, but I'm very different than him. He feels guilt, responsibility, and obligation. I just don't think he should be compelled to feel those things (I say "compelled" because it doesn't seem like he LIKES dealing with this), but don't know what other words of wisdom I can impart upon him about this topic. Why should I care? Well, it makes him bothered, cranky, and/or "off" when he's dealing with one of her "flare-ups", which makes it part of our relationship.
<bot>: Partner of 5 years is being email-stalked by a potentially mentally unstable woman and could use help/advice.