I have a 'team managed' service management project. An issue type's workflow has an approval step set to 'customer selects approver' and the approval field set to the system generated 'approvers' field.

There appears to be no options to edit the options within this field.

This option allows customers to add their own approver by specifying an email, even if they do not have a Atlassian account within our instance.

This has resulted in customers (1) entering incorrect email addresses and (2) adding users that do not have a profile within our instance, which requires an agent to manually change the approver.
Can the approver list be restricted to users (or a subset of users) with accounts on our instance?