A Domain using Firstwave was getting the below errors in the email headers :

Authentication-Results: spf=permerror (sender IP is x.x.x.x)

Received-SPF: PermError (protection.outlook.com: domain of domain.com used an invalid SPF mechanism)

Firstwave SPF record needs to be the following

“v=spf1 include:spf.firstwave.com.au ~all” for your domain(s).