@KDfromG
Not to question what you're seeing, but I have a GMX account for testing, and just tried it out... Messages did get moved into Trash in web mail.
So yes, please check the mapping between Aqua's "[ Deleted ]" and GMX's Trash folder, as linked by @mikeone:
- Long press the account -> options and folders -> folders
- Find GMX's server side "Trash" folder
- Tap it, choose "Sync with [ Deleted ]"