I'm answering this 7 years after the question was asked, but I had the same problem with SharePoint 2016 and wanted to add a couple of additional pieces of information to @Jesus_Shelby's excellent response...
- If you are not seeing a message hit your drop folder you have a routing problem. This means your SMTP server never gets the message.
(nothing to add from what @Jesus_Shelby said)
- If it's making it into the drop folder but not getting picked up - you have configuration problem. Check to ensure SharePoint knows where the drop off folder is.
Check the permissions on the Drop folder. In my case I needed to allow read/write access for the appropriate account. I think this would be the account that is running the SharePoint Timer service, but I am not sure. In my case it was a dev environment and I just gave "Everyone" Full Control on C:\inetpub\mailroot (the folder that contains the Drop folder). Looking at the ULS logs helped me realize that this was the problem as I saw an error there indicating:
A critical error occurred while processing the incoming e-mail file C:\inetpub\mailroot\Drop\112bee9301d746a200000001.eml. The error was: Access to the path '112bee9301d746a200000001.eml' is denied..
BTW, if you use ULS Viewer, creating a filter and setting Category = "E-Mail" helps filter things down to what you want to see.
Another reason it may stay in the drop folder is if the email address the email is going to does not match the email address for the incoming email setting on the library. You should see an error in the ULS logs if this is the case as well.
Errors occurred processing 1 message(s): Message ID: The following aliases were unknown: <alias>
The alias part above is the first part of the email address it was sent to (the part before the @ sign).
- If you see it get picked up from the folder - then your problem goes back to the first item with the folder rules. Mail is being delivered and SharePoint is picking it up and processing it.
The specific problem for me was that I needed to set "Save original e-mail?" to Yes. I was so caught up on "E-mail address" and "E-mail security policy" that I didn't think to look at this for a while. Clearly the others matter, but for me the "Save original e-mail?" mattered too. If the email has an attachment, the attachment will come through with this set to No. However, if there is no attachment it just does nothing (except to remove the message from the Drop folder). BTW, the Send-MailMessage PowerShell cmdlet does allow for sending one or more attachments.