Hi Greg,
This can be done using a content filter/message filter.
Condition:
Envelope sender/Envelope recipient: specialuser@example.com
Action:
Route to alternate host: mailhost99.excample.com
Do note mailhost99.excample.com should be resolvable by the DNS server to an IP address in order for it to deliver to it.
Once the above filter is implemented only emails matching the condition would route to mailhost99.excample.com while all other emails would process normally using SMTP routes or DNS MX records.
Thanks!
Libin Varghese