I ran into an issue where I was sending emails to a document library, and though I had everything set up correctly, I was getting the following message:
Delivery has failed to these recipients or groups:
The recipient e-mail system can’t process this message now. Please try resending the message later.
This was a very simple fix. I added an A record to the site, and called it nav.yesprep.local pointing it to my server, however I didn’t add it as a binding in IIS as it seemed like everything else worked. So I added it, and it worked ASAP: