Trouble Wrapper and Fixing Unwrapper
Checkers such as SPF may detect problems, and choose to bounce an email. Alternatively, it might be wrapped into an explanatory email, and perhaps allow a reply to confirm a solution.
```
This email was not sent to you directly by the original domain,
but through a number of intermediate servers, which means that
it may have been forged:
list.mydomain.com
probe.funnyrabbit.net
This sequence can make sense to you if the email arrived via a
mail server or if you used an ancient email forwarding service.
In this case, you can make an exception by replying to this
email.
The original email is attached. If you leave it on, you will
have it sent back to you.
```
The email could be sent from postmaster@mail0.example.com
and may
have a reply address with a digital signature that gets it through
to a fixing service.
Generic code: Wrapper and unwrapper SMTP daemons. Creating and verifying a fixer address. Memorising fixes and looking them up.
Specific code: Checking a condition, testing if a fix exists. If not, composing a message. If a fix is requested, remember it.