Eu acho que você precisa ativar o Mail Relay para o endereço local.
Allow relay for privileged IP ranges. Allows people with certain IP addresses to send email through the server. If the IP addresses of persons who are able to send email out through the server is known, use this option. DO NOT select this option if the list of IP addresses is unknown, as this may inadvertently allow everyone access. This option is usually required to allow sending through the server from a web server or web page.
MainEnable Administrator - > Servidores - > localhost - > Conectores - > SMTP - > Propriedades - > Relay - > Ativar reencaminhamento de correio - > Permitir retransmissão para intervalos IP privilegiados. Adicione seu IP lá (simples "127.0.0.1" deve funcionar bem).