Could you please try the following:

• First, disable MTA-STS. Click Reports/MTA-STS & TLS Reporting, then uncheck MTA-STS Enabled. Once you've done this, try sending the message again from the Outbound Queue.
  
  Let me know if the message goes through.
• Next, click on the Tools menu (the word Tools itself). The following page will display a few tools. Run a DNS Lookup on gmail.com and paste the text for the MX record here.