Modtage mails på sendmail

Ive got 27 domains running on a single IP. Alle these domains send out notification emails (kinda like forum notifications) from donotreply@domainname.com. (note : domain name changes depending on which domain is sending mail)
In some cases the email is rejected as the receipient server will try asking my server if the sender email is a valid email address (which its not).

Id like to create it into a valid email address, (so that my emails sent are not rejected) so I can receieve emails on the "donotreply" account, but the emails should be deleted and not stored.

Im using sendmail+PHP to send mails with. Any suggestions on if the above is possible and how?

I know i need to create a MX record in the DNS and open up in the firewall on port 25. Im just not sure how to configure sendmail to receive mail on account "donotreply@domainname" and throw it away.


Thanks in advance for the help


PS I må gerne svare på dansk, jeg copy/pastede mit spørgsmål fra et engelsk forum hvor jeg ikke fik nogle brugbare svar.