Hi Evan,
I did a lot of research for the “Unknown Error( #17886)” issue at my side, and found that it also could be caused at the server side. However, based on the current situation, we are not sure whether this issue is related to the 18500 error issue which is being fixed. Therefore, I suggest you check the “Unknown Error( #17886)” issue again after the 18500 error issue is fixed. I also would like to collect the tenant domain name (the free domain looks like xxx.onmicrosoft.com) via private message. If the issue persists then, I will involve the backend team to do more investigating on the “Unknown Error( #17886)” issue. I’m sorry for the inconvenience that may be caused by this issue, and thank you for your understanding. To send a private message, click Private messages under Your details on the right side of the community site.
Meanwhile, my colleagues and I will update the latest status of the 18500 error issue here for your reference.
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Microsoft is continuing to work towards resolving an issue in which some customers using Mac Outlook or Entourage may be unable to send messages exceeding a size of 6 megabytes. An update to the latest build is being performed to remediate impact. Engineers have completed the deployment for over 90% of customers and expect to finish all deployment within the next 12-17 hours. The next update will be provided by December 11, 2013 at 6:00 PM UTC, or upon service restoration.
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Thanks,
Mindy Pan