Send the mail errors back to the sender

[LecrisUT]

Currently when mailout receives a mail to redirect, the sending server will consider it successful. There are two ways mailout can be configured to resolve this issue.

1. mailout will read the header before accepting the mail, try to connect to the rule receiver, and test with the same header. If it succeeds simply pass it through as usual. If the receiver denies it pass the same error to the original sender.
2. mailout will fully receive the mail and attempt to send the mail a few times. If it fails due to instant errors: MX record is not set, receiver denies the connection, then have mailout send the errors to the sender. If the receiver cannot be contacted it should try again for the standard 24h before sending an error to the sender that it cannot be contacted. (Or even better after a few attempts send a warning email, that the receiver is not able to receive the email at the moment, and mailout will try again for some time before it declares the message failed. Maybe allow the sender to be notified when the mail was successfully relayed)

I can see merit to both configuration, and I think the user should decide which error handling to go for.