It's meant to
be generic, in theory, but we don't really say anywhere "Hey, we have
included the right magic for gmail so you just need to say -authservice
gmail". We do point to the mhlogin(1) man page, but we don't go into
great detail there either.
inc(1) man page:
which must be specified with the -authservice service switch.
Before using this, the user must authorize nmh by running
mhlogin and granting authorization to that account. See
mhlogin(1) for more details.
mhlogin(1) man page:
mhlogin currently only supports OAuth for Gmail. Run mhlogin
-user username -saslmech xoauth2 -authservice gmail
even though only Gmail is supported out of the box