Why is Exchange 2016 using DNS A record instead of MX record to send mail?

Exchange 2016 trying to send mail to sargb.com.

Instead of sending to their MX "sargb-com.mail.protection.outlook.com", it is sending mail to sargb.com (18.235.162.72).

I cleared the DNS cache on the 2-domain controllers and flushed DNS on Exchange.

What should I try next?

TiA!