Hi Evan,
We have received several reports of the similar issue about "Error code: 18500". I'd like to confirm some additional information to clarify the issue:
1. Are there any attachments when trying sending emails via Outlook 2011 for Mac?
2. Are the attachments larger than 5 MB?
3. Can these issue affected users send emails with attachments larger than 5 MB via OWA (http://mail.office365.com/)?
Best Regards,
Bruce Zhou