Hi TSmith3601,
Please click on the link below and follow the steps in Step 4 to solve the issue.
http://support.microsoft.com/kb/2492140
Please change the proxyAddresses from SMTP:user@domain.onmicrosoft.com into SMTP:user@domain.com. You can refer to the picture below. 
After performing this step, please force a synchronization to check whether the issue still exists.
Thanks,
Yang Yu