I saw interesting case in Azure DevOps which looks like some kind of limitation. When I set my business email in variable group like that:

And change applicable piece of code in this way:

stages:
    - stage: DEVApproval
      displayName: 'DEV Approval Gate'
      variables:
        - group: MY_APP
      jobs:
        - template: /manual_validation.yml@Pipelines
          parameters:
            email: $(email)

After a while I get the email to my business outlook account, so it works fine.

When I set in variable group the PDL taken from outlook (there are several people with me), I didn’t get email with approval. For testing purposes I keep my email, however we need to have at least two person for Approval Gate to accept.