If the targets that you are checking are behind an organization's firewall then it is is usually best to use the mailserver and a mail account of that organization to send the alerts. However if checking that mailserver itself is part of the checking you wish to do then an external mailserver such as gmail, yahoo, or outlook mail (although slower) may be a safer choice, to insure that you receive an alert if/when the organizations's mailserver itself goes down. (Unless you are also using the syslog option of appchkr, networkchkr, or serverchkr.) You will need to supply the SMTP url for the external mailserver in the cfg file, as well as a valid mailfrom account on it.