The usage message says `/usr/bin/roundup-mailgw [-v] [-c class] [[-C
class] -S field=value]* <instance home> [method]`, but it's unclear what
`[method]` refers to. I guess it refers to the four ways the program can
be called mentioned below in `--help`, but it's not really clear.

experienced with hg commit 4987:f47341cbe3c8

There is a second problem:
There are two variants of the documentation:
a) text in def usage() of /roundup/scripts/roundup_mailgw.py
b) ./share/man/man1/roundup-mailgw.1

b) seems to be more recent.

And a third one: 
the "<instance home>" part is optional as well.

Fixed the --help text with hg4990:63574f1f3570 
and opened Issue2550885 for the "second problem"
and other implications.

Thanks Karl-Phillip!