«Address in mailbox given  does not comply with RFC 2822, 3.6.2»
To prevent a bug with swiftmailer and an unset returnpath, use the fromEmail for the return path; else we may get the error message «Address in mailbox given  does not comply with RFC 2822, 3.6.2».
As a quick fix, I used a copy of the fromEmail, emails are sent normally. However, side effect includes a mail notification to the emailSender for each subscriber openning the newsletter.
the controlform needs a field to set that email (to the desired no-reply@domain) or real email address.