Noticed that you were using the "Microsoft Exchange Online" tag, so suspecting you were using the Exchange Online email account for work or school, right?
As mentioned by Patchfox, how did you send the mail, via OWA(https://outlook.office.com) or from Outlook desktop client?
If you were using the Outlook desktop client, please have a check in web mail and see if the mails can be received properly. If there's no issue in Outlook on the web, which means it's an issue with the desktop client, then it's suggested to create a new Outlook profile and readd the email account to check the result:
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