it took me some time to get the voicemail to email working so I want to share what went wrong if anyone runs into the same problem:
UC320w with software 2.2.3 (5), but same problem was there with the last GA.
I send the mails to my own mail-server (Postfix) where the final receipients are. Because there is no mail-relay in this setup I did not configure a username and password as it is not needed when mails are sent to local receipients.
In this setup the Mails were not sent as an attachment, but were scrambled as ... well, shown in the picture.
I experimented with and without TLS, with different ports my mailserver is listening on but no success.
The only thing that helped was to enable authentication by providing a username and password. For the functionality of SMTP it is not needed, but the uc320w probably is not aware of that ...