This happens because you are using the php mail() command - which then sends it through the smtp server listed in the php.ini file.
However, when it gets to that server, it's saying "hey, you're not
sending to me - you're sending to an external address. I'm not going to
let you".
You can do one of two things to resolve this:
- change the smtp server settings to allow relaying from the server's ip address
- set up & use 'smtp authentication' which should allow you to
send anywhere. This will most likely be set up on the smtp server
already so you just need to go to the Interspire Email Marketer settings page and put
in the appropriate details in the 'SMTP Server' section.

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