Yes, you have to use the new address. Last time I used one of the old-
addressed mailers (last year), it took a couple of months because they
weren't getting stuff forwarded to them from the old address any more,
and they only rarely went over and checked for it.

While I hate to say anything negative about MPL - they generally do
very nice work in a very small specialty niche biz that must be tough
to keep going - I think it is EXTREMELY unprofessional of them not to
make more of an effort to check for mail sent to the old address. If
there are still significant numbers of mailers with the old address
out there for sale or in customer's hands (and they probably know how
many there are, they number the damn things!), those are still "good"
mailers, and the onus is on THEM to make sure they still check for
incoming mail to the old address.

So... Although I think it's RIDICULOUS that MPL doesn't recognize that
it's really THEIR responsibility to support mailers sent to the old
address, it's up to you to make sure that you send it to the right one.