I would ask myself:
-- When the email from z comes in, how likely am I to remember to
send the other emails?
-- How soon after the email from z comes in do I need to send the
-- How important is it?
I would then set up a repeating reminder to myself. It might be a piece
of paper placed in my system such that I'll see it approximately every 2 days.
(I would only take about a second or two to look at it each time.)
It would remind me that when the email from z comes in, I have to send
the other emails. After seeing that reminder every 2 days, probably when the
email comes in I'll remember to do it immediately. If not, I would do it
when I next see the reminder. I would also put the email addresses both
on that reminder page and somewhere else where I could find them when I
look for them, probably in my general computer-file of email addresses and stuff.