Você pode configurar o Postfix para aceitar tais endereços com a opção allow_min_user = yes
.
A documentação do postconf também explica por que o padrão é rejeitar esses endereços:
Allow a sender or recipient address to have '-' as the first character. By default, this is not allowed, to avoid accidents with software that passes email addresses via the command line. Such software would not be able to distinguish a malicious address from a bona fide command-line option. Although this can be prevented by inserting a "--" option terminator into the command line, this is difficult to enforce consistently and globally.
As of Postfix version 2.5, this feature is implemented by trivial-rewrite(8). With earlier versions this feature was implemented by qmgr(8) and was limited to recipient addresses only.