smtp;550 5.7.1 Your message was rejected due to example.com’s DMARC policy. See https://support.apple.com/en-us/HT204137 for info
The sent message failed the DMARC policy for the sending domain. If the sending domain has a reject policy set for DMARC, the message must pass DKIM or have SPF aligned to the domain with a custom Return-Path. To resolve this, you'll need to contact your email service provider to see how to send a message that passes DMARC support.