EmailSubmission/set should fail when user do not exist

[chibenwa]

Handle this as a Mail -> boolean abstraction EmailSubmissionSetValidation.

Idea is to optionally plug controls for synchronous validation

• Does the recipient exist ?

The idea would be to reproduce the synchronous failure behaviour of SMTP ValidRcptHandler.

Sample conf

email.submission.set.validations=LocalRecipientExistsValidation,...

Implicit FQDN

[chibenwa]

SetError

invalidRecipients - The rcptTo property of the envelope (supplied or generated) contains at least one rcptTo value, which is not a valid email address for sending to. An invalidRecipients String[] property MUST also be present on the SetError, which is a list of the invalid addresses.