He will become cold and aloof and disconnect from the author of the letter, the former having lost its supply source status by daring to post such a letter. The reaction would be no different to a verbal confrontation unless intimidation is involved. If intimidated, the narcissist will recoil and become submissive, overly sentimental, dependent and idealizing. Otherwise, the reaction would be as above. I posted on the circumstances of the life crisis that led to my self-realization and the post can be found in the list archives.
All generalizations are false.
I discussed the narcissistic couple in one of my FAQs . Narcissists can be happily married to submissive, subservient, self-deprecating, echoing, mirroring and indiscriminately supportive spouses. They will also do well with masochists. The Narcissist makes use of this naivete and exploits it to his benefit. The natural protective mechanisms which are provoked in normal people by love - are cold bloodedly used by the narcissist to extract yet more narcissistic supply from his writhing victim.