Answer by Kier for Edit user's email field in Samba domain

Open the user config for editing (default is vi, but you can specify alternatives, such as nano with the --edit argument

samba-tool user edit username

Add the following line:

mail: newemail@domain.ext

substituting the desired email address for newemail@domain.com

Then save and exit.I worked this out by creating a new user with the requisite --mail-address= argument and opened that user's config to see what field was added for email.