Create source does not offer full options

When I Create Content > Mailhandler Source the form for collecting the particulars of the source no longer gives the full options. Eg there is no question asking for the mailbox. Please see screenshot.

It used to be there. Now it isn't. It even disappeared from the edit view of a source that I had previously created.

Any ideas please?

Using D7.50
I'm trying to set up Mailhandler to import emails from a Mailman mailing list, and display them in threads in a forum. I've not got anywhere near getting it working. The mailbox knows there are messages in the inbox.