Well if it's PHP you need to modify, locate your php.ini file..
Setting extension-specific variables
Extension-specific variables are stored in separate sections of the configuration
file. For example, all the variables related to the MySQL extension should be in
the [MySQL] section of the php.ini file.
If you're going to use PHP's mail() function, there are three variables you may
need to set. The SMTP and sendmail_from variables (on Windows) or the sendmail_
path variable (on UNIX) are used when sending e-mail messages through PHP's mail()
function. On Windows, these variables set the SMTP server to be used and the
From: address to display in e-mail messages; on UNIX, the sendmail_path variable
sets the path of the MTA (mail transfer agent) for mail delivery:
SMTP = myserver.localnet.com
sendmail_from = firstname.lastname@example.org
sendmail_path = /usr/sbin/sendmail
I am guessing you are running a web site on that machine and it can no longer
send emails to people for notifications, etc ? ?