Sending and Receiving mail with SMTP failed

I can’t receive and send external emails from exchange.

I have Exchange 2007 on Win 2008 Server with AD. My domain name is xxx.com is hosted with an external hosting company thr which i receive and send mails using POP3 (mail.xxx.com).

I have an internal domain with xxx.com (same with external domain). i have configured the exchange to send and receive mails internally, but i cant receive and send external mails.

I have configured the send and receive connectors in the hub transport but not sending or receiving mails.

please i will appreciate if this can be assisted with

thanks