There should be a option for default email domain for LDAP users.

Use case: large organizations often have multiple object trees, and some might not contain the users email address explicitly. In some cases you might have a default email domain for all users (example.com) in addition to their unit-specific (unit.example.com) domains.

For more dynamic settings like a Django settings.py you might to able to specify that the email address of an users is their uid + the default email domain of your organization, but when that's not the case you instead relay on the settings being provided to you, as would be the case of Mattermost.