There is a bug where, under certain conditions (usually related to 2007-2010 upgrade or when a server name changes) a list will become 'stuck' where it cannot receive email. The fix is fortunately extremely simple and painless :
- Go to the list properties and select 'Incoming Email Settings'
- Copy the current email address to someplace else, like Notepad++
- Change the 'Allow this list to receive e-mail' to 'No' and hit 'OK'
- Go back into the incoming email settings for the list
- set the 'Allow this list to receive e-mail' to 'Yes' and paste the
original address back in and hit ok.
- Go back into the incoming email settings and make a note of the FULL email address required. Usually this is something like 'firstname.lastname@example.org'
- Send email to the address as it appears in SharePoint
If that doesn't work, check the permissions on the list and make sure that whoever is testing the email functionality has at least 'contribute' permissions to the list. Either that or enable the 'Accept e-mail messages from any sender' option, though this is only advisable in a controlled Intranet environment.