The man page link has more info. Did you see it?
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Also, you can optionally prepend a user name and password combination for authentication to each mail server. The format of the username and password pair is user1@mymailhost.example.com. User will be prompted for the password. The username and password can be specified on none, all, or some of the mail hosts.
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When you execute the modify command, if you specify a user as above, you will be interactively prompted for a password.
Note1: AutoSupport only supports simple authentication. STARTTLS is not supported. There is an RFE in the system for that with no target date.
Note2: With ONTAP 9.5 and later, AutoSupport configuration is enforced cluster-wide. So, the user:password configuration for the mailhost is shared by all nodes in the cluster.