My email server are refusing mails coming from outside..

[webstower]

Hi,

If I send an email from info@cancerconciencia.org.ar from webmail to info@cancerconciencia.org.ar it was delivered fine, The mail appears as new in the inbox.. But, no mail are coming from outside.. gmail for example, tries to deliver a mail to that mailbox for a few days.. and then, return with the legend: "I gived up.."
In other words.. no mail are coming from outside the local server..
Any ideas were to start? maybe something about exim configuration?
Thanks for any help in advance!