If you cannot send or receive emails you may want to check to see if you have received any "bounceback" messages at the sending address.
These are messages sent from the server containing your original message that was being attempted for delivery with an error code.
If this error code is in the 400's it means that this is a temporary error and can usually be retried again later.
However, if this error code is 550 it means that this is a permanent error because the user's mailbox was unavailable, so resending it later will also fail.
Typically a 550 error message indicates that the email address that the message was sent to does not exist or cannot be found.
If this is an email account that you have recently created on our servers check that the Email Routing is set to Local Mail Exchanger.
More details are contained in the following Knowledgebase article: What is the difference between Local, Backup, and Remote Mail Exchanger for Email Routing?
Some typical 550 error responses are as follows:
550 - Verification failed
550 - Relay not permitted!
550 - Invalid sender
550 - No Such User Here