SQL Server: can dynamic data masking be safe when providing a database backup?

I have a SQL Server 2016 SP1 with data that I am ready to share with a partner (developer), except for a few columns, example: email varchar(64) MASKED WITH (FUNCTION = 'email()') NULL I give the partner the credentials for logging in to the test copy of the database as a certain user, without unmask permission. I understand this is reasonably safe. Now, we are considering sharing the database backup with our partners, so they can reload in their own server and environment. I would probably need to have contained users, so that partner cannot create new users or connect them to existing logins (not sure about this). CREATE USER user_name WITH PASSWORD = 'strong_password'; Is there a way that I can share a database backup, and still trust that the masked columns are safe, or do I have to physically delete the data?