make civicrm work without a bounce channel

Hi,

The past few zillion errors on the forums are due to people not configuring the mailbox.

I suggest
0) out of the box, the mailbox isn't default
1) if no default mailbox, use the sender email for the return-path/envelope
2) in the wizard (in step 2 add a warning "CiviMail not configured to handle bounce emails or replies, bounces will return to the sender address"+link to the doc.
2b) or 3 next to the from "CiviMail not configured to handle bounce emails, bounces will return to this address
3) don't display in step two the replies tracking
4) add a CIVICRM_NO_RETURN_CHANNEL in civicrm.setting.+ explanation that it isn't the ideal solution. If set don't display the "not configured" error

I'm on vacation when you are back, do what is the best, but at least we need to properly warn people that their mailbox isn't configured