550 5.4.1 Recipient address rejected: Access denied.
https://answers.microsoft.com/en-us/msoffice/forum/all/550-541-recipient-address-rejected-access-denied/3291aa18-4116-4e89-be31-0d77d194bcb7
WEBAug 8, 2023 · The message gets rejected because there's no valid object in Office 365 that matches the SMTP address of the recipient in Exchange Online. Note, DBEB is enabled when the domain is set to Authoritative. If the domain is set to Internal Relay, messages to invalid users are rejected and senders receive a 550 5.1.1 …
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