Hi Jesse,
To troubleshoot the issue, I’d like to confirm the following information with you:
1. Does the issue also occur in OWA?
2. Is the recipient internal or external address?
The post you mentioned is using Powershell to achieve the requirement.
For more information about how to Use Windows PowerShell to connect to Exchange Online, please go to the following Microsoft website:
Connect Windows PowerShell to the service
http://help.outlook.com/en-us/140/cc952755.aspx
Note, please run the PowerShell as administrator.
Thanks,
Jason Jiang