Your domain authentication may be weak
Mailbox providers want proof that your email is allowed to use the domain in the message. Missing or broken SPF, DKIM or DMARC can make even legitimate email look risky.
- SPF should include the service that sends your mail.
- DKIM should sign messages with your sending domain.
- DMARC should exist on the visible From domain.
- Alignment should make sense between From, DKIM, and return-path domains.