Sendmail how to forward specific accounts to another server - email

Question

I have to linux mail servers running sendmail, both handling the same domainX.
server1 is declared as MX.
server2 is intended as a supplement that has more storage
I want server1 to handle all email for domainX as follows:
user1: handled locally by server1
user2: handled locally by server1
user3: relayed to server2 and handled locally by server2
How can I achieve this with sendmail?
I know I can forward all mail for domainX to another SMTP server, but how do I do this for only some userY#domainX, but not all ?

Answer 1

Found a workaround, it works for me, not entirely sure if it handles all possible situations properly.
server1 handles mail for domainX
server1 is declared with MX record in DNS
server2 handles mail for domainX and server2
server1 has user1, user2, user3
server1 has user3
server1 has in virtusertable user3#domainX user3#server2
server2 has in virtusertable #server2 %1
The scheme works as follows:
user1 and user2 are handled locally by server1 according to default configuration.
user3 is a virtualuser, translated to user3#server2, thus no longer processed locally, regardless of the existing local user user3.
server2 accepts mail for it's own hostname (specified explicitely).
All mail for all users #server are translated to local usernames. Mail for user3#server2 is translated to local user user3.
Of course, using prefixes to differentiate local accounts between multiple domains is possible.