When attempting to login as VDS mail user, either from the Site Manager (E-mail/FTP Management) or directly from /users/ I get a runtime error.
This is a known bug with 3.7.
When creating a mail user with ftp disabled, the user is being created without any home directory.
That would also cause some mail issues for that user.
Create the user with ftp enabled.
This will create home directory for the user.
You can disable FTP to the user afterwards.
This problem will be fixed in SD3.7 UP1.