Failing "Use Classic Email as Transport" on core28

when trying to use a classic email as transport, this fails using core28, getting the following error:

<img width="374" height="796" alt="Image" src="https://github.com/user-attachments/assets/1f2d079c-a44e-4c88-be7d-6767e2d4f98b" />

i double checked that the account as such is working on a core25