Change "root" to "root.admin" in outgoing e-mails

Our AIX servers send e-mails which have the "from" address set to "root@company.com" for our root user ("C{M}company.com" in /etc/sendmail.cf). The problem is that when bad e-mails are sent out or rejected by remote servers, they are being returned and delivered to e-mail box of "Mary Root".

How can I change/alias/masquerade out going e-mails to look like they are coming from "root.admin@company.com" when root sends e-mails?