[otrs] OTRS is not sending e-mails

Hello I am facing the following problem: No mails are sent of any event to the agents. The error I get in the log is the following: [Mon Sep 21 16:07:10 2009][Notice][Kernel::System::Ticket::Article::SendAgentNotification] Sent agent 'NewTicket' notification to 'igutierrez@inmagusa.com'. [Mon Sep 21 16:07:10 2009][Error][Kernel::System::Email::SMTP::Send][116] Can't use from '': 550Requested action not taken: mailbox unavailable ! Enable Net::SMTP debug for more info! I think I am missing the definition of an e-mail to be used, but where should I define it. Thanks, Agustín de Landa