ZyXEL Communications VPN Series Handbook page 760

Note: Must make sure SMTP Server configuration is correct otherwise message
will unable send to SMS provider successfully.
Setup Email to SMS Provider configuration
Go to "Configuration > system > Notification > SMS Select "SMS Provider" as Email
to SMS Provider. Enter SMS Provider Email server domain name.
And configuring sender mail address in "Mail From"
Note: Your SMS provider has to allow the email address which configured in "Mail
From" to prevent the email is denied by SMS provider's mailbox.
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