Currently, this is a manual process. If you are using sendmail (I don't know about other mailers), then change to /var/spool/mqueue
# vi qf* find a line like: S<the email address you send with>
It will be just before a line starting with RPFD:<recipient's address>
Change the address in the S<> line to whatever is your vsnl email address. Everything else can remain as it is. Mail should go through correctly.
I am still trying to figure out how to get sendmail to do this correctly.
Philip