Basically, you want a filtered Email address policy at this point.
Use the EAC to replace the existing primary email address for a
filtered set of recipients To create additional email addresses that
will be used as the primary email address for a filtered set of
recipients, follow these steps. In the EAC, navigate to Mail flow >
Email address policies, and then click Add Add Icon. On the Email
Address Policy page, complete the following fields: Policy name
Enter a unique, descriptive name. Email address format Click Add Add
Icon. On the Email Address Format page that appears, make the
following selections: Select an accepted domain Click the drop-down
list, and select the new authoritative domain. Email address format
Select the appropriate email address format for your organization.
Select Make this format the reply email address. When you are
finished, click Save. Run this policy in this sequence with other
policies Typically, policies that apply to specific users should
have a higher priority (indicated by a lower integer value) than other
email address policies, including the default policy. Specify the
types of recipients this email address will apply to Select the
recipient types to which you want the email address policy applied.
Create rules to further define the recipients that this email address
policy applies to Click Add a rule to restrict the recipients that
this policy will apply to. This creates a Boolean And statement.
Repeat this step as many times as necessary. CautionCaution: If you
apply too many rules, it’s possible to restrict the email address
policy to the point that it doesn’t contain any users. Click Preview
recipients the policy applies to to view the recipients that policy
will apply to. Click Save to save your changes and create the policy.
You’ll get a warning that the email address policy won’t be applied
until you update it. After it’s created, select it, and then, in the
details pane, click Apply.