Not able to relay from Exchange Online

I have a domain ohprs.org whose email is managed by exchange online. I have another host on a subdomain, dc1.ohprs.org. This host is physically separate from the ohprs.org hosts and is a Linux host running sendmail.

In exchange, I have a forward set up for myuser@ohprs.org to mfoley@dc1.ohprs.org. When I send an email from my personal computer to myuser@ohprs.org, it does not deliver to mfoley@cd1.ohprs.org. In Outlook, for the account myuser@ohprs.org, I get the bounce message, "myuser is not authorized to relay messages through the server that reported this error." On the dc1 host maillog I get:
Code:Nov 28 23:16:34 DC1 sm-mta[2505]: 3AT4GYCm002505: ruleset=check_rcpt, arg1=<mfoley@dc1.ohprs.org>, relay=mail-mw2nam10hn2205.outbound.protection.outlook.com [52.100.157.205], reject=550 5.7.1 <mfoley@dc1.ohprs.org>... Relaying deniedHow do I permit mail from Exchange to be delivered to DC1? Note that I cannot simply put 'mail-mw2nam10hn2205.outbound.protection.outlook.com' in /etc/mail/access as a RELAY because the "2205" bit changes with each message.

Note that if I forward from myuser@ohprs.org to some other host, e.g. mfoley@novatec-inc.com, it goes through without problem. So, it must have something to do with the FDQN dc1.ohprs.org.

More information on the Outlook bounce message:
Quote:

More Info for Email Admins Status code: 550 5.7.367 The sender's message was routed to an email server outside Office 365 that returned an error that it can't relay the message. It's likely that the server isn't set up correctly to receive and relay messages sent from ohprs.org or from hprsadmin. The receiving server is not an Office 365 server - it's likely one of your on-premises servers in a hybrid environment, a server at a smart host email service that you're trying to route messages through, or possibly a server at an email hosting service you used in the past (for example, when your MX record still points to your previous email service provider). To fix this issue, try one or more of the following: Check the error shown in the "Error Details" section below - Check the error to determine what domain, service, or server is reporting the error. The email server needs to be configured to either accept messages from anonymous users or to include the sending domain or sending IP address in its list of authenticated senders. On an on-premises Exchange server, you can set this up in the server's receive connector. If it's a smart host managed by another service or partner, contact the service or partner and ask them to configure their servers to accept and relay messages from your domain or from your senders. Make sure your MX/DNS records are correct - Work with your DNS hosting provider (your domain registrar, Web hosting provider, or ISP) to make sure your MX records are correctly set up to direct messages to the correct location. Unfortunately, Office 365 support can't help fix this problem because it's likely due to misconfigured settings on an email server outside Office 365.