Resposta encontrada: você precisa configurar o MDaemon para aceitar mensagens endereçadas a (ou vindas de) usuários locais desconhecidos, que são recusados por padrão: link .
Account Verification
SMTP MAIL address must exist if it uses a local domain
Click this option is you wish to verify that the MAIL value passed during the SMTP process points to an actual valid account when it is purported to be from a local domain or gateway.
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SMTP RCPT address must exist if it uses a local domain
Click this option is you wish to verify that the RCPT value passed during the SMTP process points to an actual valid account when it is purported to be from a local domain.