Search the forum for 'email alerts' This problem can have multiple sources, virus software which prevents the sending of email, (like McAfee8.0 does, its defaul behaviour is to block port 25), incorrectly configured SMTP server etc..
You can test it to try to telnet to port 25 of your SMTP server from your backup server and see if you can get a connection.
Regards
A van der Boom