1. It really should have a 'test connection' operation. Otherwise its quite tricky to see if you've got it right. If you got it wrong it'll be slowing down every single cronjob

2. I think we could do something much more intelligent with messageinbound_mailbox, messageinbound_domain, messageinbound_hostuser

For the majority of people, they need their mailserver, username, password - we can work out the rest from the username. Have an advanced operation for those cases where its not like that.