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The IP you're using to send mail is not authorized to 550-5.7.1 send email directly to our servers

Situation: when sending email from a test Exchange server using dyndns.org DDNS and Comcast as ISP, you may receive this message: mx.google.com #550-5.7.1 [x.x.x.x] The IP you're using to send mail is not authorized to 550-5.7.1 send email directly to our servers. Please use the SMTP relay at your 550-5.7.1 service provider instead.

Resolution: It is possible you can setup a send connector on Exchange so that you can send mail using Comcast smart host. To do that, please follow these steps to setup the send connector:
1. Go to ESM>>Organization Configuration>>Hub Transport.

2. Select “Send Connectors” tab.

Exchange Send Connectors

3. Right click and choose New send connector
4. Name it and set intended use to “Internet” click next

Setup Exchange Send Connectors

5. Add new “Address space” put a * in the address and select “Include all subdomains” click next

Setup Excahnge end Connectors

6. Select “Route mail through the following smart hosts:”
7. Click add and select FQDN type in smtp.comcast.net click ok and then next.

Setup Exchange Send Connectors

8. Choose “Basic Authentication” put the username and password of your Comcast account click Next.
Setup Comcast SMTP Relay

9. Make sure your exchange server is listed here as the source server

10. Send email to test it.

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