If it's not too late, just delete the mail. Or read it, then delete it (just in case it's an olive branch or another kind of closure).
I know how easy it is to fall in that kind of "ignoring", but in my experience it's usually a sign that you need to do something for yourself and throw out all "should"s.
And it being someone's birthday should never be a reason to do something you don't want to. If she's reckless enough to send an email to someone she had a messy relationship with on her birthday, she should be able to handle if it doesn't pan out well. You can be friendly, polite, detached, not answer, or even be rude, depending on her mail and your relationship, and her birthday shouldn't factor in.
I understand not wanting to be cruel or rude when there's happiness to destroy, but if someone kicks a hornet's nest, they're just asking for it.
Of course, I'm a total cynic about birthdays, so my perspective may be a little off.
Pretty much. I've been reading way too much Doctor Nerdlove, it seems
No matter what, if she's that willing to fool around with you behind her boyfriend's back, she is not girlfriend material. Seriously. Planning sex with someone who is not your boyfriend is just crossing all kinds of lines.
Edit:
@Vella: If I can be of any use, feel free to write