Almost three years later and I'm now looking for an answer to this question.
Actually, I'll settle for mail coming from outlook simply respecting the 'zimbraPrefFromAddress' attribute.
I've set it using:
zmprov ma
zt2@example.com zimbraPrefFromAddress 'ztest2@example.com'
and now mail coming from the webmail client uses the desired alias by default, but when using outlook and connector version 6.0.5902.6, mail arrives at its destination as
zt2@example.com.
Any hints?