It doesn’t appear to be possible with the mail action.
You could set up the Mail action with one email provider and sendmail or the like with another email provider and use executeComandLine to send through that avenue. But u don’t think there is any way to find out that the email failed so you wouldn’t know when to come the backup option.
Personally if the first email option fails is fall back to a different sheeting mechanism like Prowl or Telegram. But the problem of discovering that the email failed is still a problem