Try using this:
_api/web/lists/getbytitle('listname')/items(100)?$select=Initiator/EMail,RAReviewers/EMail&$expand=Initiator,RAReviewers
Above is working fine for me in case of person or group.
Example:
/_api/web/lists/getbytitle('TestList')/items(1)?$select=TestPersonField/Title,TestPersonField/EMail&$expand=TestPersonField
Output: Below is part of response returned from above API call
<entry>
<content type="application/xml">
<m:properties>
<d:Title>Ganesh Sanap</d:Title>
<d:EMail>[email protected]</d:EMail>
</m:properties>
</content>
</entry>
<entry>
<content type="application/xml">
<m:properties>
<d:Title>SPConnect Members</d:Title>
<d:EMail>[email protected]</d:EMail>
</m:properties>
</content>
</entry>
Where SPConnect Members
is a Microsoft 365 group name.
Note: This will not work for SharePoint groups as they don't have Email property.