Copy the majordomo file to /etc/postfix/majordomo from its backup location in /root/sendmail-old/majordomo. Then make sure that the following line is uncommented in /etc/postfix/main.cf:
#alias_database = hash:/etc/aliases, hash:/opt/majordomo/aliases |
Then change this: hash:/opt/majordomo/aliases to this: hash:/etc/postfix/majordomo and type newaliases. You will also need to change your majordomo aliases if you were using bulk_mailer. You should convert them over to includes. See section 6 of the Red Hat Postfix HOWTO for detailed information on using majordomo and using aliases with Postfix.