People are trying to email me and getting a 550 5.1.1. message

Hi Hilary,

Based on your description, this issue may not be limited to a single sender. Since people trying to email you are receiving a 550 5.1.1 error, you are not receiving new messages, and your outgoing emails are also delayed, the problem may be related to your mailbox or mail flow configuration.

If it is possible, you may ask the senders who are receiving the 550 5.1.1 error when emailing you to try the following:

• Verify that they are sending messages to the correct email address and that the address has been entered correctly.
• Remove your address from their Outlook Auto-Complete list and enter it manually again. This can help if Outlook has cached an outdated recipient entry.
• Compare the recipient address shown in the NDR with the address they intended to send to, to make sure the message is not being redirected to an invalid address.

Additionally, you can check whether any email rules in your mailbox have been configured to automatically forward or redirect incoming messages to another email address. If so, correcting the destination address or removing the rule may help prevent 5.1.x delivery errors.

Also, you may wish to contact your IT administrator and ask them to follow the administrator troubleshooting guidance in the article below:

Fix NDR 550 5.1.1 to 5.1.20 in Exchange Online - Exchange | Microsoft Learn.

If you are unsure who your IT administrator is, you can refer to this official Microsoft guide for assistance: How do I find my Microsoft 365 admin? - Microsoft Support.

Hopefully this helps point you in the right direction. Feel free to reply back if you need further assistance.