There was a misunderstanding as I was unaware of duplicati-cli. Now everything is clear.
To sum up, the --send-mail* (and most likely all --send* commands?) are job-specific, not server-specific. When I think about it now, that makes perfect sense.
I added the default options for my jobs and now everything works as expected. Thank you!
The comments about help were a bit off-topic here, but then, my first assumption about the help prompt was correct, they weren’t listed there so they were not meant to work.
Finally, a suggestion from me. In the documentation, I am still missing a clear distinction between the options for duplicati-cli and the ones for the duplicati-server. Maybe it is worth putting a note somewhere there to avoid similar confusion.