Having that form of validation should help since DKIM was made to specifically stop email spoofing; and anyone serious about implementing Portier would understand that need.
Outlook/Gmail etc aren't likely to be flagged by ESP's for the most part as those have reputation and rudimentary spoofing protection, at least more than email@example.com anyway.
Persons using custom domains are usually the admins of those domains, and Office 365/GApps/Registrars etc. provide simple ways to enable DKIM.
In the past, there was an SPF RR type but that's been deprecated.