What is MAIL_ENCRYPTION and how do I find out what I should set it to?

What is MAIL_ENCRYPTION and how do I find out what I should set it to?

From the variable description, "Encryption level from your email provider" the email provider used would typically include this in their documentation. Should I add some clarification to the description to make this clearer and possibly reference that this is typically set 'tls' for most providers?

What is tls?