It would be good to display the "mail to" address for an ideas portal on both the Ideas Overview page (accessible to 'reviewers') and the portal itself.
As described, the Mail to address for idea portals is not promoted on the Ideas overview page or portal itself. Because of the relative ease of adding ideas from either area, we currently do not have plans to make changes at this time.
However, we would like to point out that all email notifications can be customized via Account -> Configure idea portals -> Email branding. This can be a potential place to promote the additional functionality around emailing ideas (which could also be a timely touchpoint).
We hope this helps.