In versions of the Form Mail NG plugin prior to v4.0 (at least v3.x) it was possible to administer several mail-form macros in one Confluence page and submit those forms independently. This option seems to have gone missing with the paid upgrade to v4.0.x
The current behavior is a bit unpredictable, but it seems all mail-macros in a Confluence page are processed even if they reside within different mail-forms.
Is this a bug or does the plugin no longer support this feature?