Build a Sender Policy Framework DNS TXT record with provider presets (Google, Microsoft 365, Brevo, Mailgun, SendGrid, Postmark, AWS SES, and more).
Toggle every provider that legitimately sends mail for your domain.
v=spf1 mx include:_spf.google.com -allDNS publication
Create a TXT record at @ (apex) with this value. Only ONE SPF record per domain. SPF has a 10-DNS-lookup limit — count your includes.