Exchange Server 2010 won't recieve emails from Public Servers

I have almost done setting up Exchange server and run into a little Rock in the Road and need some assistance..

first off this is the first time I have set up exchange so I am kind of GREEN if you know what I mean.

I have successfully:

1. installed
2. added mailboxes with domain controller accounts
3. set up sending connecter and can send to any public server.

where I run into the rock in the road is receiving emails from public server. when I send an email from a public email to a mailbox on the exchange I get a response of:

"The error that the other server returned was:
550 5.7.1 Unable to relay"

I guess I needed guidance on where to go from here... I am constantly researching and can't seem to come up with any answers...

feel free to point me in the right direction.

thank you very much

 Curtis