which way do you send out the Mail in the Workflow? Did you select a user manually or are you using a global variable? Best practice here would be to send mails to a global variable, for example: user 1 has a task to assign someone who has to proof-read some document. He has one field in the workflow form which lets him choose a user from a group. this form field then gets written into a global variable of type "User from a group".
Next up is an E-Mail notification to the global variable, which now holds the username. DocuWare then looks up the E-Mail address which is configured in the user account and sends a Mail there. If no mail address is found, the mail is sent to the workflow user you configured in the "Start" action of the workflow.
My guess is: 1. that you are either trying to send to an E-Mail address directly
or 2. you are trying to send to a user retrieved from an index field of the document which may not work because it does not specifically hold a username, only plaintext
or 3. you have no Mail address defined in the user settings. to solve this, either log in to DW as the user in question, click on the username on the top, then my profile, then fill in the Mail address. Or: log in to the Administration Desktop app, switch to: Organization -> User management -> Users -> the user in question -> fill out the mail property in the right pane. Remember to save!
Hope this helps.
Greetings from Germany,
Simon H. Hellmann