This problem has been noticed for sites with external email servers and sendmail disabled. The problem exists because of checks introduced in sendmail to give an outright "User Unknown" error message when trying to deliver to a non existing user on a domain. The previous behaviour was to first accept the mail then do a check locally and bounce back with a user does not exist mail if the user did not exist in the domain. The change was made to prevent unnecessary mails in the system which happened when spammers tried to guess usernames on sites using brute force.
The specific directives in the sendmail.cf file are
Kdomainmap hash -o /etc/virtualhosting/mappings/domainmap.dband the following directives for each site
Ksite1_a hash -o -a -m /home/virtual/site1/fst/etc/aliases.db
Ksite1_u text -o -a -f -z: -v2 /home/virtual/site1/fst/etc/passwd
This however does not take into consideration the situation where the mail is hosted on another server.
To get past this, Edit the site and enable sendmail on it. Then make sure you have a catch-all account set for the domain. Edit the site to disable sendmail on the server. Make sure that the domain aliases file /home/virtual/domain.com/etc/aliases.db exists after you have disabled sendmail for the site.