For mail that you're sending yourself, from your own infrastructure (e.g. the envelope-from address matches the header-from address), DMARC + SPF should be suff

For mail that you're sending yourself, from your own infrastructure (e.g. the envelope-from address matches the header-from address), DMARC + SPF should be sufficient and is easy to implement. For third parties that are sending on your behalf, you'll likely need DKIM - but that will be implemented on their side, and all you'll have to do is add the DNS record they give you.