No RFC822 Address Possible in the Person Document
Since IBM Lotus Notes/Domino Version R5.x, the Internet mail sender addresses are stored in the person document in the directory (= NAMES.NSF on the Domino server).
Setting the Internet sender address in the person document
The field help (accessible by clicking the static field label) explicitly points out that no display name (phrase) is allowed here.
A sender address in the form "Manfred Dillmann" is therefore not permitted in the "Internet address" field.
Solution
The desired form of the sender address can be defined in the Domino server's configuration document.
Relevant settings in the server configuration document
In the RFC822 phrase handling field, the default selection shown is Do not add phrase. In addition to various other options, there is also the option Use CN as phrase (CN = Common Name, the user's first and last name from the person document).
After restarting the router task, emails should be sent in the desired format.
Note
This setting in the server configuration document does not change the Internet mail sender address in the users' location document.