translating outward message address?

Microsoft keeps putting my machine onto its blocked list (and no, it is
NOT being used for spam or for denial attacks or...) I suspect someone
does not like me and keeps complaining to MS about my address. 
Anyway, some of my people I communicate with have alternate addresses. I
would like to have postfix translate an address like
[email protected] to [email protected] since the user has a working gmail
address, and gmail does not block my address.
But I do not know how to get postfix to rewrite the address. (I often
get mail which is CC: to [email protected] amongst others, and if I reply I
often forget to translate the address ( and besides, that is what
computers should be for, doing trivial tasks better than I can) and it
then bounces.

I thought maybe transport would work, but if so, I have no idea how to
do it.
[email protected]   [email protected]
or
[email protected]  smtp:[email protected] 
in /etc/postfix/transport do not seem to work.

Putting the latter into /etc/postfix/relocated just bounces it back to
me, and does not deliver it to [email protected]