Backport #26267 by @wxiaoguang
1. Fix the wrong document (add the missing `MODE=`)
2. Add a more friendly log message to tell users to add `MODE=` in their
config
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Backport #26068 by @felixvictor
The setting `MAILER_TYPE` is deprecated.
According to the config cheat sheet, it should be `PROTOCOL`.
Co-authored-by: Felix Victor <felix.victor.na@gmail.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
