You can use the @mention
control in pull request discussions, commit comments, changeset comments, and shelveset comments. You can also use the @mention
control in any text field on the work item form.
To @mention a user you've never selected previously, just continue typing to perform your search against the full directory.
Upon completion of your selection and text entry, your @mention user will receive an email alerting them about the mention.

More details please refer official tutorial: Use @mentions
In your case, you could test some other scenarios such as pull request/commit comments and other users if they could receive e-mails. Also check if the mail be blocked as spam in mailbox.