Hello Jonathin
This is not a bug, is the intended way Blesta Support Manager only chnages the "Reply To" email adress, noting else But maybe @Paul//@Tyson/@Cody can also confirm this on core Support Manager
I have try to replicate the issue and, it works fine for me
Do you have "debug mode" active on your blesta config file?
Can you try to use PIPE, insted of IMAP, to see if it works?
Regards,
PV