I had a customer with a primary SMTP domain but a couple others for certain individuals within the business. The SSL cert was a wildcard bound to the primary SMTP domain and Autodiscover/ Outlook configuration would fail for other SMTP domain logons.
I read various articles on changing IIS for redirects etc
However I found the simplest solution was to remove the autodiscover A record for the secondary domains (internal & external) and replace with a single SRV record pointing back to the primary domain.
Port Number: 443