Cannot recieve outside email

[Lindzbob03]

I have 4 new sales reps and they have not been able to receive outside emails for the past week. They can send email fine but when customers reply, the message does not go through. The customer gets a message similiar to this:

Delivery status : Failed. Message could not be delivered to the domain - name.com. Failed to accept the recipients.

Any suggestions on how to fix this problem quickly?
Note: Everyone else has not been having the problem just the new accounts. They could, however, recieve emails when the account was created, but the problem just started occuring.
Thank you.

 

[Lindzbob03]

The problem has been fixed.

 

[ReadRight]

What was wrong?

 

[Lindzbob03]

They were not put into the firewall (sidewinder). They told me they could recieve outside emails before, but that is highly unlikely.