Changing Email Address Policies in a hybrid configuration
Our default email policy has our domain in all caps (@DOMAIN.COM). When we create a new user and the email gets created in EXO the user shows as mailto:bo@DOMAIN.COM. I know I can go to the email address policy using the on prem exchange server that we are only using for changes (no mailboxes exist on that server). When i click apply is gives me a message about how this will take a long time to apply if you have a lot of users and if you have more than 3000 users to use the EMS (we only have about 500 users).
My concern is, will this start changing things for existing users or removing anything for existing users? For example, will i now see mailto:bob@DOMAIN.COM and mailto:email@example.com (as an alias/secondary email)?
Check your default domain in EXO under DNS section. If your default domain is set to bo@DOMAIN.COM then any new user creation will have that email address or will default to bo.
Email address policy will be applied in EXO not from on-premise server.