Exchange Server Question

We have setup an exchange server 5.5 and it’s been working well. The only trouble that we are getting is that when we send out more than one email at times we get a bounce back with the following message:

Subject: Undeliverable: test

Your message did not reach some or all of the intended recipients.
The following recipient(s) could not be reached:

‘test on 4/16/02 11:48 AM
No transport provider was available for delivery to this recipient.

Sometimes its able to sent sometimes not. I’ve checked the microsoft site to look for help, didn’t provide much usefulness. Please let me know if you have any insight on this problem. Thanks!