sendmail & vacation

[Greg Groth]

Are there any tutorials available on how to set up vacation to operate 
with a whitelist?  My users are requesting I add this functionality to 
our mail server, but I don't want to create an open bounce.  What I wish 
to do is take a list of domain names of our customer base and allow 
vacation to only respond to that list.  Googling brought back info on 
using whitelists to use in conjunction with spam blocking, but I haven't 
been able to weed through all of the results regarding spam to locate 
anything regarding whitelisting vacation.  From the vacation man page, I 
see that there is an -x option that allows blacklisting, but nothing on 
whitelisting.  Any ideas?

Also, if this functionality is not available using vacation in 
conjunction with sendmail, would a different MTA offer this functionality?

TIA

Greg Groth