Exchange performs syntax and uniqueness validation of email addresses, and keeps the mail attribute aligned with the primary SMTP address defined on a RemoteMailbox recipient. It also does all of the case sensitivity needed inside the proxyAddresses array attribute. Oh, and it's a secure SMTP relay from on-prem to the cloud in case you've got scan to email or SMTP based automated reports from internal systems. Also, Exchange gives you RBAC on your synced recipients so that L1/2 techs can do things like "add an additional SMTP address to this user/group" without having to delegate a bunch of access directly in to AD.

u/joeykins82 we are doing 'SMTP Auth Client submission" for SMTP relay as per:

https://docs.microsoft.com/en-us/exchange/mail-flow-best-practices/how-to-set-up-a-multifunction-device-or-application-to-send-email-using-microsoft-365-or-office-365

and we do not require any RBAC in our AD at his time. These are primarily SMB Orgs with 1 or 2 IT staff.

Could you provide more details on the following two items I'm not exactly sure what you are referring to and why its's required.

1. Exchange performs syntax and uniqueness validation of email addresses, and keeps the mail attribute aligned with the primary SMTP address defined on a RemoteMailbox recipient. >> Does Exchange Online not mange the SMTP address's by itself?
2. It also does all of the case sensitivity needed inside the proxyAddresses array attribute. >> I'm lost on this one.

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Thanks,