you have the following scenario.
Two user accounts:
Now one of the two users sends an email to that DocLib and the question is: Will this email arrive and be stored in the DocLib?
Short Answer: NO!
The email arrives at the SharePoint Server with the sender-email-address firstname.lastname@example.org and due to the security settings SharePoint will look for the user because SharePoint uses the user’s name for “Created By” and also that user has to have the right permissions to create new items in that DocLib. With the email address email@example.com SharePoint will find two users and that is not expected because to set the right user-info into “Created By” we need a unique email address. If such a Sender Email Address results not into a unique user we will have a security violation and should not store this email into the DocLib.
You can set the E-Mail security to accept emails from everyone and the result for a non-unique email address will be that you can see “System Account” as the “Created By”.
In ULS log you can find:
Windows SharePoint Services
An error occurred while processing the incoming e-mail file C:\Inetpub\mailroot\Drop\xy37ll3331cd05e80006666.eml. The error was: Access denied. You do not have permission to perform this action or access this resource..
An error occurred while processing the incoming e-mail file C:\inetpub\mailroot\Drop\36ab155201cd06af00000002.eml. The error was: Access denied. You do not have permission to perform this action or access this resource..
Enable and configure e-mail support for a list or library: http://office.microsoft.com/en-us/sharepoint-server-help/enable-and-configure-e-mail-support-for-a-list-or-library-HA010082307.aspx?CTT=3
We have to respect security and the result here is one of much more scenarios when an email will not arrive in the DocLib at the end.