My ex used to send letters like this to me fairly often. She still will leave phone messages which contain "hot button" phrases that are clearly there to try to set me off. I used to take the bait all the time.
Now, I ignore them. To the extent I can, I treat my relationship with my ex as purely a business relationship, with "co-parenting" as the subject. If it strays from that, I don't respond. If she gets "personal" or tries to attack me, I either don't respond or, if a response is required (for example, a financial obligation needs to be paid for a child's activity), I respond in a totally businesslike (read: unemotional) way.
This way of dealing with your ex, in addition to keeping the focus on only essential items related to the children, has the added benefit of totally pissing them off.
Face it, he wrote that letter as some sort of catharsis. A "tit for tat" or, "she hurt me, now I'll hurt her", "poor little ol' me" type of attitude pervades. Think about an otter, and how water slides across its back - such are the words of your stbx, touching you, but not affecting you.
Or, if all else fails, "sticks and stones will break my bones but names will never hurt me."
