There are hundreds of config options.

Without a complete design, the "config" page shouldn't show anything more.

What users really need is to lint the config file and find mistakes, but not read a page which is filled by hundreds of config options again.

From my testing, theFROM field is required to run dummy and sendmail modes. So I think it would be better to add it.

I've also noticed that theUSER field, which should be exclusive to SMTP, is mistakenly displayed in all modes.

At least, we might want to consider fix both of these.

From my testing, theFROM field is required to run dummy and sendmail modes. So I think it would be better to add it.

You should have been warned, the error log is clearer than the unclear UI. Even if you show FROM on the UI, reader doesn't know that what it should be.

I've also noticed that theUSER field, which should be exclusive to SMTP, is mistakenly displayed in all modes.

There are far more mistakes, not only this one. Fixing or not, maybe there is no difference, unless there is a complete design to resolve the problem fundamentally

Understood. Thank you for the clarification.