`smtp_sasl_security_options`

This parameter specifies the security options that the Postfix SMTP client uses for SASL authentication when connecting to a remote SMTP server.

The list is a comma-separated parameter of one or more of the following items:

- `noplaintext`: disallow methods that use plaintext passwords
- `noactive`: disallow methods subject to active (non-dictionary) attack
- `nodictionary`: disallow methods subject to passive (dictionary) attack
- `noanonymous`: disallow methods that allow anonymous authentication
- `mutual_auth`: only allow methods that provide mutual authentication (not available with SASL version 1)