See explanation below
Explanation:
In the Exchange admin center, go to Mail flow > Remote domains.
Click + Add a remote domain. The Name the domain screen appears.
In the Name text box, enter a descriptive name for the domain.
In the Remote Domain text box, enter the full domain name. Use the wildcard character (*) for all subdomains of a specified domain, for example, *.contoso.com.
Click Next. The Email reply types screen appears.
Define the following settings:
- In the Out of Office reply types section, specify which type of out-of-office replies should be sent to people at this domain.
- In the Automatic replies section, specify whether you want to allow automatic replies, automatic forwarding, or both.
Click Next. The Message reporting screen appears.
Specify whether you want to allow delivery reports and non-delivery reports by checking the respective check boxes.
Click Next. The Text and character set screen appears.
Define the following settings:
- In the Use Rich-text format pane, specify whether to follow each user's message settings, or whether to always or never preserve RTF formatting. Selecting Never means that RTF messages are sent as plain text or HTML.
- In the Supported Character Set pane, specify which character set to use (if the message doesn't specify the character set) by choosing from the MIME character set or Non-MIME character set drop-down list.
Click Next. The Review screen appears.
Review the remote domain settings, and click Save.
Reference:
https://docs.microsoft.com/en-us/exchange/mail-flow-best-practices/remote-domains/manage-remote-domains
https://docs.microsoft.com/en-us/exchange/troubleshoot/email-delivery/understand-troubleshoot-oof-replies