This almost has some overlap with MDL-48800

Here we want to check that outbound emails should work as much as possible without being able to test inbound checking.

So just assert the state of these 3 settings:

$CFG->noemailever
$CFG->divertallemailsto
$CFG->divertallemailsexcept

And also expose these in the gui (in readonly mode?)

We could possibly also send an email and just look at the error values but in practice that does almost nothing so I think is pointless.