Comment 2 for bug 266288

Mailman is not alone in rejecting messages that start with a
"-" - some MTAs also do that, for instance, Postfix (but see
Postfix's "allow_min_user" option).

The reason is that too many sites mistake such addresses for
sendmail command-line options, because most sendmail users
are clueless and forget the "--" before the addresses.

For that reason, it is rather unwise to use mail addresses
that start with a "-" - while legal, it's not universally
accepted.