with an alias file you cannot specify the domain name an alias applies for, it applies for any domain name that sendmail is server mail for, so an alias file is used for aliases that are common accross all domains, or if sendmail is only accpeting mail for one domain name.
a virtusertable is really only usefull if you are accepting more than one domain name, with a virtusertable you can have alias be specific to a signle domain name, so webmaster@foo can be forwarded to something different than webmaster@bar is forwarded to.
there is also a small semantic differnce between the two files, the aliais file has a : between the the original addr, and the forwarding add, a virtusertable file has whitespace between the two addresses.
for more information on virtusertable check out http://patriot.net/~scoile/isp-redhat/sendmail/virtusertable.
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