I received a call from a customer last week stating that they were able to configure incoming email on some of their lists/libraries, but not on others. When pressed for details they elaborated that they had enabled it in Central Administration:
however when they navigated to List/Library Settings, the option to enable incoming email under the Communications section was not present. After looking through several settings it turns out that the lists where the incoming email option is missing are custom lists.
The incoming email link will only be shown on out of the box (OOB) lists/libraries. If you have a custom list and want the link to display you will need to create a custom handler.
You should still be able to enable incoming email by navigating directly to the emailsettings.aspx page by replacing listedit.aspx in the URL. First navigate to the list in question and then to list settings. You should get a URL similar to the following:
In your URL, replace listedit.aspx with emailsettings.aspx and that should take you to the email settings page for the custom list.
Even with the workaround I don't think you can actually enable the custom list to receive incoming emails since there is no email handler for it. I know there is a 3rd party tool http://www.anurasoftware.com/ that let you enable custom list to receive incoming emails.