Error sending email in event notification

[gperes]

I have configured to send e-mails when a new ticket is created, but it doesn't send the e-mails.
My installation is with Debian. And it was made by apt-get.
in /var/log/messages i have the following:
"Sep 23 11:32:16 squid OTRS-CGI-10[16231]: [Info][Kernel::System::Ticket::TicketCreate] New Ticket [2014092310000137/hrthtrtrhtrht] created (TicketID=17,Queue=Rede,Priority=3 normal,State=new)
Sep 23 11:32:24 squid OTRS-CGI-10[16231]: [Info][Kernel::System::Email::Send] Error sending message
Sep 23 11:32:24 squid OTRS-CGI-10[16231]: [Info][Kernel::System::Ticket::Article::SendAgentNotification] Sent agent 'NewTicket' notification to 'xxxxx@xpto.com.br'.
Sep 23 11:32:32 squid OTRS-CGI-10[16231]: [Info][Kernel::System::Email::Send] Error sending message
Sep 23 11:32:32 squid OTRS-CGI-10[16231]: [Info][Kernel::System::Ticket::Event::NotificationEvent::_SendNotification] Sent customer 'NovoChamado' notification to 'xxxx@xpto.com.br'.
Sep 23 11:32:39 squid OTRS-CGI-10[16231]: [Info][Kernel::System::Email::Send] Error sending message
Sep 23 11:32:39 squid OTRS-CGI-10[16231]: [Info][Kernel::System::Ticket::Event::NotificationEvent::_SendNotification] Sent customer 'NovoChamado' notification to 'xxxx@xpto.com.br'.
"

on system configuration --> Framework --> Core::Sendmail I have the following:

SendmailModule: SMTP
SendmailModule::CMD: /usr/sbin/sendmail -i -f
SendmailModule::Host: ISP_address
SendmailModule::Port: 25 (My ISP support this port)
SendmailModule::AuthUser: xx@company.com.br
SendmailModule::AuthPassword: xx_password
SendmailEnvelopFrom: xx@company.com.br

I don't know what else to do. Please someone can help me?