DNS Issue?

To provide any specific answers in this case will require knowing the domain.

Based on your description, I recommend switching to an origin-less hostname and a redirect rule.

  1. Create a AAAA record (and no A record) for webmail
  2. Set it to the special address 100::
  3. Use a redirect rule to send visitors to https://outlook.com/example.com

You can adapt example 4 in the following guide.