Error message: 5.7.1 r281RhQs007944 This message does not comply with required standards.

Hi - I'm using Outlook 2011 for Mac and when I reply to some emails or accept meeting invites, I receive the following error message:

Error:

5.7.1 r281RhQs007944 This message does not comply with required standards.

Details:

Mail could not be sent

Account name: my email account

Error code - 17099

I'm new at using Outlook for the Mac so if anybody can help troubleshoot this issue that will be greatly appreciated.

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