requirement of message filter

[Vishal6]

Hi All,

Followed below document where it states need to create a message filter for header to send outbound mails. Can i know what impact if i dont create it.

https://www.cisco.com/c/en/us/support/docs/security/cloud-email-security/214812-configuring-office-365-microsoft-with.html

[Ken Stieers]

This message filter is put on the cloud ESAs to check if mail coming from MS's servers has a custom header that you set in your O365 tenant.
Otherwise ANY O365 tenant could be configured to send mail through your CES instance and it would send it.

[Vishal6]

thank you for your guidance. So it means its  optional not an mandatory ?. 

[Ken Stieers]

Not really optional.
If you don't put it in, your CES instance is now an open relay for anyone on O365.
It's the only way to make YOUR O365 tenant "authenticate" to the CES instance that its YOUR tenant, not mine.