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Unable to send emails using Office 365 SMTP connector


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It appears that we had/have quite a few reports of emails that fail to be delivered using an Office 365 connector with the following error:

554 5.2.0 STOREDRV.Submission.Exception:SendAsDeniedException.MapiExceptionSendAsDenied

This error is not a Hornbill error, but an error returned by Office 365. For the reason mentioned in the error (SendAsDenied) the email will not be sent. We cannot offer assistance with this type of error as it is not caused by a fault in Hornbill functionality or Hornbill configuration. There are numerous articles online discussing this error and from our understanding, the error means the user whose credentials you specified in the SMTP connector cannot submit messages on behalf of the user specified in the From/Sender MIME headers (source). In other words, if you have a Hornbill mailbox with an email address as mailbox@yourdomain.com and you have an Office 365 SMTP connector having username credentials like example@yourdomain.com then you would need to add mailbox@yourdomain.com as Send-As permission on example@yourdomain.com Office 365 account or alias if you are using that (source).

The above does not represent a bulletproof solution, it is just a suggestion of a possible solution for this issue. Please be mindful that we (Hornbill) do not offer any Microsoft Office 365 expertise, therefore, we strongly advise you to seek assistance with Microsoft for this error if any suggested solutions (here or elsewhere) does not fully resolve this for you.


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