I've had this issue before. Problem was resolved for me by setting up SPF records in DNS.

In any case, the key to resolution is to inspect the bounce back messages.

Forward to me directly if you like.

From: "Warren Toomey via TUHS" <tuhs@tuhs.org>
To: tuhs@tuhs.org
Sent: Wednesday, May 3, 2023 7:46:51 PM
Subject: [TUHS] OT: TUHS mails not going to Outlook & Hotmail. Help!

All, e-mails from the TUHS server are not making it to Hotmail or Outlook.
I've not changed anything. Is there anybody with some MTA/ISP experience
who might be able to help diagnose the problem?

Thanks, Warren