What this means in practice
They are distinct DNS hosts with their own records and keys.
Before changing anything, identify which hostname each provider uses so one setup does not overwrite the other.
Practical guidance
Keeping an existing mail provider is possible because a subdomain and the apex domain are separate DNS hosts. You can configure IaGenify on a subdomain while the apex continues to use the records and keys for your current provider. The two configurations do not overwrite one another simply because they share the same registered domain.
Before setup, identify the exact hostname you plan to use and leave the existing apex mail records in place. Add or change records only for the subdomain assigned to IaGenify, and verify the host shown in each record. This separation is useful when you want to test a new mail flow or use a distinct service without migrating all mail at once. If you later move providers, plan that change separately and update only the relevant DNS records.