We have seen cases lately, where on some SharePoint 2010 environments (version: 14.0.6120.5000 > meaning April 2012 CU) the Administrator has followed the steps listed in the following article:
Configure incoming e-mail (SharePoint Foundation 2010)
http://technet.microsoft.com/en-us/library/cc287879
Altgough the steps have been applied correctly, in order to configure the incoming emails on Libraries or lists, the emails do not get delivered!

When the user sends an e-mail to a list, the e-mail is received by SMTP and saved to the SMTP drop folder “C:\inetpub\mailroot\Drop”, bit it stays there and they don't get collected by the Microsoft SharePoint Foundation Incoming E-Mail timer job. TheOWSTIMER is running and the output of this cmd command: STSADM -o getproperty -url http://YourSiteURL -pn alerts-enabled, is the correct one:

PS C:\Users\admmadis> STSADM -o getproperty -url http://YourSiteURL alerts-enabled
<Property Exist="Yes" Value="yes" />PS C:\Users\admmadis>

While researching the ULS logs, you will notice some interesting lines like the following: 

05/24/2012 08:59:34.99         OWSTIMER.EXE (0x0220)                           0x15E8        SharePoint Foundation                 E-Mail                                6873        Warning         An error occurred while processing the incoming e-mail file C:\inetpub\mailroot\Drop\eb5080f101cd38ca00000001.eml. The error was: Unknown alias: ###..        7698697e-d356-4c53-9ab0-f9b9e7ab58b1

 05/24/2012 08:59:35.01         OWSTIMER.EXE (0x0220)                           0x15E8        SharePoint Foundation                 E-Mail                                0000        High            E-mail cannot be delivered because site is over quota or locked for editing. Site URL: http://YourSiteURL.        7698697e-d356-4c53-9ab0-f9b9e7ab58b1

 05/24/2012 08:59:35.03         OWSTIMER.EXE (0x0220)                           0x15E8        SharePoint Foundation                 E-Mail                                6871        Information        The Incoming E-Mail service has completed a batch.  The elapsed time was 00:00:00.0312514.  The service processed 6 message(s) in total.  Errors occurred processing 6 message(s): Message ID:  Message ID:  Message ID:  Message ID:  Message ID:  Message ID:    The following aliases sent more than five messages: http://YourSiteURL (6 messages)  The following aliases were unknown: ###         7698697e-d356-4c53-9ab0-f9b9e7ab58b1

Following these errors, you need to apply the following steps to the SPS2010 server and hopefully the issue will be resolved:

Go to the SharePoint Central Administration > Application Management > Configure quotas and locks > on the Site Quota Information section >  set a limit (i.e: 5000 MB) on this setting:  “Limit site storage to a maximum of:”  > and then press “OK”.
By that time and on,  the “stuck” emails should be collected - from the Drop folder  by the Microsoft SharePoint Foundation Incoming E-Mail timer job and you will see the incoming emails working again.