Hi @Mariyah Alcantara ,
I understand that you sent the messages from Outlook client, but it returned an error message from system administrator, is it right?
In this case, what's the type of your account (Exchange on premises, Exchange online or others)? And what's the version of your Outlook client(File>office account>About Outlook), please first ensure that you have updated to the latest version.
Did the issue occur when you send the messages to any specific recipients only or all recipients? And are you working with the external network now (such as outside your domain of your organization)?
In order to further confirm if the issue is related to your mailbox or your Outlook client, if it possible, it's recommended to log in to your web mail to send the messages for testing.
For the Outlook client, I found a relevant document that mentioned '550 error' may occur when you work with external network and not configure the smtp setting normally in Outlook, for more information and cases, please refer to this document: I receive a 550, 553, or relay-prohibited error when sending email messages. In this case, if you are working with IMAP or POP account in Outlook, it's recommended to check your outgoing server setting like below(control panel>mail>email account>double-click your account>advanced setting>Outgoing server>check the option "My outgoing server(SMTP) require authentication")
If that's not your case, please provide us with more information about it(such as full information about the error message, account type and client version etc.)
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