Sendmail Failing to Send to Local Domain

I’ve run into this issue a couple of times now so I’m assuming others are as well. ┬áServers set up on Amazon EC2 with default sendmail configuration seem to get the following error in their /var/log/maillog: 12 Sep 19 19:48:26 DOMAIN sendmail[31192]: q8JJmQQV031192: … User unknown 13 Sep 19 19:48:26 DOMAIN sendmail[31191]: q8JJmQ0D031191: [email protected], […]