Is it true that "Incoming email setup" is only available for document libraries and not custom lists?
If so, how do I enable it for a custom list (100)?
Out of the box SharePoint only supports receiving email to specific types of lists and libraries. For instance, you can receive email on an Announcements list. However, you cannot receive email on custom lists.
You could, however, start with an announcements list and add your custom fields as necessary.
There's no out-of-the-box built-in method for handling email to anonymous lists. The reason is fairly simple; in a custom list, no columns but the Title column exist by default, so there would be no way to take the contents of an incoming email and put into the list.
However, that doesn't prevent you from having custom lists with email if you're willing to do some custom development to help SharePoint map incoming email to your list. The event receiver SPEmailEventReceiver accepts an email as its parameter and fires whenever someone sends an email to your list.
When you have attached the receiver to your list, you can access the email options on the list settings page for the list. It will still be your responsibility to set up the mapping of email properties to columns in your list.
Also, remember that incoming email is handled by a timer job, so you need to attach to the OWSTimer process to debug your code.
I wrote some example on how to accomplish this in Building the SharePoint User Experience. I think it was Chapter 6, and I'm sure you can find a copy somewhere :-)
This is not true, you can enable incoming emails on a custom list. Check this post out:
Your custom list basically needs to implement the EmailReceived event handler.