If you were currently in a committed relationship, and an ex (spouse, boyfriend, girlfriend, whatever) started emailing you and getting more and more flirty, would you allow yourself to flirt back?

would you flirt with the ex, or would you nip it in the bud and not engage in flirting emails? if you found out that your current spouse or mate (or whatever you call him or her) were engaging in flirtiness via email with their ex, would it upset you? or would you think it harmless?