[TASK] Move isAllowed check to importer class

To make the isAllowed part extendeble this is moved
to the importer class so you can create custom
checks depending on the import class to assure
an email is allowed.
[FEATURE] Add option to send notification on succes/failure

It is now posible to send a notification through email
if a mail is rejected and/or a mail was succesfully
processed.