Outlook / SMTP error code 554
smtp;554 5.2.121 The sender has exceeded the limit for the number of messages they can send to this recipient per hour. For more information go to http://go.microsoft.com/fwlink/?LinkId=526653. [#.prod.outlook.com #]
The sender has exceeded the maximum number of messages they're allowed to send per hour to a specific recipient in Exchange Online.
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