Inform us if a mail address is invalid

When setting up a manual booking and thus writing the guests email manually, a mistyping will not be known for us hosts until the guests contacts us about never getting any information.

We don’t know that because hospitables automated messages are shown as sent, even though / if the recipients email provider sends back a message telling that the user does not exist. All these returning mails (user not existing, recipient mailserver not responding/existing) should be caught and forwarded to our chat / inbox.