Migration of Email domain - will this break original azure AD sync

Question

I am moving two email domains to my office 365 tenancy as part of a merge.
The original company is using one of these email domains for sign in to Azure for SAML based access.
Will this break access to these resources once I verify the domain, or is email and Azure AD separate?

We will not be changing anything on the original domain and ADDS sync still occurs.

Answer 1

It's best to think of the Office 365 suite as features on top of Azure AD, in the same way that Exchange Server (on-prem) runs on Windows Server Active Directory, Exchange Online, Sharepoint Online, etc all run on top of Azure AD.

Domain verification is shared between Azure AD and Office 365. Removing it from the Office 365 admin portal or the Azure AD admin portal will have the same outcome. This will force the removal of that domain suffix from the values of userPrincipalName, mail, and proxyAddresses. This will in turn impact ability to sign in to applications if the userPrincipalName has changed, if the application has a dependency on the userPrincipalName having a certain value that your domain removal will change.