Relaying is the process of sending a message from one point to another through an intermediary (using the SMTP server you have been assigned to send e-mail messages to recipients whose addresses belong to other server). Any mail server that supports this should have some kind of filter system in place to avoid unauthorized use by spammers. Mail servers without this protection are open to hijacking.
In order to be granted to relay you have to be properly authenticated or at least allowed to do it. So maybe:
1.- you need to perform a POP authentication first. Select POP from the authentication pull-down menu.
2.- you are using POP authentication rather than ESMTP. Switch to ESMTP from the settings and try again. Once properly authenticated your SMTP server will likely relay all external addresses.
3.- your server doesn't use ESMTP but an allowed Domains/IPs list. The server will not relay any message from a domain or IP not listed in its allowed hosts/IPs list. This happens when you are not connected to the internet through the right service or in the case your server is local, you are not properly listed in the server allowed hosts list (contact the server administrator).