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Typically this means that the mailserver is disallowing the use of the mailbox. As observed https://stackoverflow.com/questions/45725630/ews-connection-to-office365-fails-401-unauthorized it may be the case you need to request through your mailhost admin that the related mailbox to be specifically accessible for this app

Duplicate Notifications with Microsoft Graph

This issue stems from the communication flow between the Outbound Mail client requesting to send a mail using Microsoft Graph.

After sending the notification, we expect to receive either:

• A 202 Accepted response, indicating the request was successful

• A 429 Status Code, along with a Retry-After header indicating when the next request should be attempted.

We have (at the time of writing) an open ticket with Microsoft regarding the problem, that the request is returning a 202 Accepted response and a Retry-After header value. Due to this external bug, our solution has been to currently inhibit the retry attempts, as the initial request to send the mail is successful.

Harmless problems

Some problems are transitory, there is nothing to typically do in these cases as connections are retried, examples of problems that can typically be ignored are:

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