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How do you set up hosts and custom routes IN g suite to get traffic to flow through to Office 365?

We are currently migrating from Office 365 to g suite for the majority of our users.
For now, we need some of the traffic to run through google and forward back to Office 365.

In gsuite, this option is Apps -> G Suite -> Settings for Gmail -> Advanced Settings
There are 2 sections that have to be setup.
Custom Route

Here is an article where they discuss this: https://support.google.com/a/answer/2614757?hl=en
My guess is that I think I need to concentrate on the "Host / Add Mail Route" part.

I've attached the screen so you can see what it looks like.
The only guess I could make was to enter:
outlook.office365.com : 587

For the custom routing, I'm telling it to use the host above and to send all email for johndoe@domain.com -> johnedoe@domain.com (at Office 365)
When I test this, I get the following error:
The response from the remote server was:
530 5.7.57 SMTP; Client was not authenticated to send anonymous mail during MAIL FROM [DM5PR2201Cxyz.namprd22.prod.outlook.com]

Does anyone have any suggestions? This seems like it would be a simple implementation because of all the Office 365 -> g suite and g suite -> Office 365 migrations that people are doing but obviously I'm struggling!

Thanks in advance!!
Randy A
Randy A
1 Comment
MichelangeloIndependent ConsultantCommented:
Try using the Office 365 MX host for your o365 domain which  should be xxx.mail.protection.outlook.com port 25 ( find exact value for xxx in your o365 dashboard under Domains )

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