The Eureka card is the only other possible Zodiac communication addressed in near identical fashion to the 2001 offering (shown in tandem with the envelope below). The problem arises when we consider that the Eureka card was not discovered until March 3rd 2007 by editorial assistant Daniel King of the San Francisco Chronicle among some photo files. The 2007 Zodiac movie directed by David Fincher and based on the Robert Graysmith book, was released in the USA the day before this correspondence was unearthed, on March 2nd. The movie had its premier in Los Angeles and California on March 1st.
This means that whoever wrote the January 10th 2001 card, addressed it in near identical fashion to a previously unreleased Zodiac communication. The Eureka card wasn't made public until 2007, so the author of the 2001 communication with near certainty had to be the author of the December 1990 Christmas card. You cannot 'copycat' the handwriting or style of an envelope that hasn't been released into the public domain. If you believe the Eureka card to be genuine, it follows that you should believe the 2001 communication to be genuine also. If you hold the opinion that Zodiac was still alive in 1990 mailing Christmas cards, then it is extremely likely he was still active in 2001 doing exactly the same thing.