Rocky Linux 8 with sendmail hosting multiple domains works fine. Only one domain should be configured to use a smarthost with a third party anti-spam service. We want to prevent that every e-mail will be relayed to one external smart host, all other e-mails should go out directly.

1. Create a smarttable file.

   # touch /etc/mail/smarttable

2. Download smarttable.m4 and copy the file to /usr/share/sendmail-cf/feature/

   # cd /usr/share/sendmail-cf/feature/
   # wget http://jmaimon.com/sendmail/anfi.homeunix.net/sendmail/smarttable.m4

3. Add following entry to your sendmail.cf file.

   FEATURE(`smarttable')dnl

4. Rebuild the sendmail config

   # /etc/mail/make
   # service sendmail restart

5. Create your smarttable

   # cat /etc/mail/smarttable
   @domain-a.com relay-cluster-eu01.hornetsecurity.com
   @domain-b.com relay-cluster-us01.hornetsecurity.com

6. Rebuild smartable by using makemap

   # makemap hash /etc/mail/smarttable < /etc/mail/smarttable