I presume you are following these Mac OS X Server: Microsoft Outlook clients may not be able to send mail (TS3023) instructions.
Unfortunately, anything (such as chgflags) that might make the configuration file immutable risks causing other weird problems in some subsequent OS X Server update or Server.app changes that expects to alter the configuration file.
I usually use the following variant, rather than editing the file directly:
sudo postconf -e 'smtpd_helo_restrictions = reject_invalid_helo_hostname'
sudo postfix reload
You could conceivably add the postconf command into the system startup. (Though I'd consider grep'ing the results of postconf -h smtpd_helo_restrictions and flagging the change, rather than automatically making the change automatically.)