If you ask Siri to search your inbox and you get results, you can review them in the 'conversation' scroll area > 'mail' card. However, if you want to open one in the Mail app, normal VO commands will not work to do so.
Bug First Encountered
How often the bug occurs
Route the mouse to the message you want to open with vo-cmd-f5, then double click (with the mouse, or vo-shift-space).
In my case, the mouse pointer can not be moved to the VoiceOver cursor position, thus I am not able to use the suggested workaround. If I press VO-Command-F5, the mouse pointer just keeps staying where it was before starting Siri.
I even have cursor tracking enabled (VO-Shift-F3), where the mouse pointer should follow the VoiceOver cursor automatically, but it does not within the Siri conversation scroll area. Elsewhere the mouse pointer follows the VoiceOver cursor without a problem.