Vanity email - 3rd party authentication

Summary:

Has anyone set up vanity email that uses a 3rd party email authentication? If so, what service provider did you choose when setting up the SPF policy?

Content (required):

Has anyone set up vanity email that uses a 3rd party email authentication?

Version (include the version you are using, if applicable):

23A

Code Snippet (add any code snippets that support your topic, if applicable):