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)
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.