If a keyboard shortcut is defined for an "Input Action" (Trinity Control Center -> Regional & Accessibility -> Input Actions) and the triggering key is XF86Mail, then pressing the XF86Mail button opens the email client regardless of what action is assigned to it.
If the action is deleted or assigned to a different shortcut, the mail program does not open when pressing XF86Mail.
This started happening a few days ago after an apt-get upgrade on Debian.
# dpkg -s tde-trinity Version: 4:14.1.3-0debian12.0.0+0
-- Davide