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>
(cherry picked from commit 72b55c8094)