Solaris

Following are the steps I took and it works after the changed.
Masquerade mail as domain.com instead of hostname.domain.com.

1. cd /usr/lib/mail/cf

2. cp main.mc sendmail.mc

3. vi sendmail.mc

Add the following items:

MASQUERADE_AS(`domain.com')dnl
MASQUERADE_DOMAIN(`domain.com')dnl
MASQUERADE_DOMAIN(localhost)dnl
MASQUERADE_DOMAIN(localhost.localdomain)dnl
EXPOSED_USER(`root')dnl
FEATURE(`masquerade_entire_domain')dnl
FEATURE(`masquerade_envelope')dnl

4. /usr/ccs/bin/make sendmail.cf

5. cp sendmail.cf /etc/mail/sendmail.cf

6. /etc/init.d/sendmail restart


Note:
Modify /etc/mail/sendmail.cf

# "Smart" relay host (may be null)
DSmailhost$?m.$m$.