Por padrão, o Postfix anexará o domínio do servidor ao final do correio enviado localmente. Isso é descrito aqui: Parâmetros de configuração do Postfix: append_dot_mydomain (padrão: yes)
append_dot_mydomain (default: yes)
With locally submitted mail, append the string ".$mydomain" to addresses that have no ".domain" information. With remotely submitted mail, append the string ".$remote_header_rewrite_domain" instead.
Note 1: this feature is enabled by default. If disabled, users will not be able to send mail to "user@partialdomainname" but will have to specify full domain names instead.
Note 2: with Postfix version 2.2, message header address rewriting happens only when one of the following conditions is true:
The message is received with the Postfix sendmail(1) command,
The message is received from a network client that matches $local_header_rewrite_clients,
The message is received from the network, and the remote_header_rewrite_domain parameter specifies a non-empty value.
To get the behavior before Postfix version 2.2, specify "local_header_rewrite_clients = static:all".
Você pode alterar sua regra de encaminhamento de e-mail adicionando um ponto final ou desativar append_dot_mydomain
em main.cf.