We are not receiving external emails.

Hi @Niall O'Sullivan

First, you can use a personal account and send an email to your business account. When the emails got blocked, the server will return a non-delivery report (NDR), if you have it, please provide it.

If the whole organization cannot receive external emails, please contact your IT admin and verify these things:

• Please confirm that the MX record points correctly to Microsoft 365 and that the domain is fully set up. An incorrect or missing MX record will prevent external mail from reaching Microsoft entirely. For reference: Connect your domain by adding DNS records
• Please go to Exchange Admin Center (EAC)> Recipients> Mailboxes, select and open your mailbox> Mailbox features> Message delivery restrictions> Require that all senders are authenticated. If you want to receive messages from external senders, this option should be unchecked.
• In EAC, check Transport Rules: Look specifically for rules with: “Sender is outside the organization” Actions like: Delete message, Reject message.
• In EAC, check message trace: If message trace shows emails as delivered but they are not visible, an admin should also check spam policies, quarantine, and mail flow rules that may be filtering external messages. For more information, please refer to: Message trace in the new EAC in Exchange Online | Microsoft Learn

I hope these information helps, please double check and keep me updated.

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