Hi Stefano,
Thank you for pointing me in the right direction.
The reason is that the mail server complains about: Mail with no From header not accepted here.
The solution is found in the reply address:
Specify the e-mail address of the person responsible for the administration of the AXIS 70U. This address will be used as the sender when scanning to e-mail destinations, unless the user specifies another sender at the moment of scanning. If authentication is used the e-mail address of the authenticated user will be used instead.
Kind regards,
Bob