SharePoint 3.0/2007 doesn't support custom list to receive emails. It only only supports enabling and configuring incoming email for next types of list
To enabled incoming email for Issue list, Task list, Custom list of SharePoint 2007 or SharePoint 3.0,
please see a solution at Anura Software ( http://anurasoftware.com )
| Anura Email Enabled List for SharePoint 2007 |
 | Anura Email Enabled List extends the ability of receiving incoming emails to all types of SharePoint list. Microsoft SharePoint 2007 limits this ability by only allowing receiving incoming emails by Document Libraries, Announcements list, Calendar list, Discussion board and Blog list. Anura Software helps you to remove this limitation and extends this ability to all your selected lists, such as task list, issue tracking list, and any custom lists. Anura Email Enabled List also contains outgoing email features which can send confirmatoin and notification to email senders. This is useful for building SharePoint based issue tracking system. Please read Build A Email Enabled Issue Tracking List. | Details | Trail | Order |
Anura List Email Address Viewer Web Part for SharePoint 2007 |
 | Anura List Email Address Viewer Web Part is a free web part to help you to publish SharePoint incoming email enabled list's addresses to your end users in a central location. This web part is a free web part and does not require Anura Email Enabled List for SharePoint 2007. | Details | Free | Free |