Sometimes a message "bounces", meaning it cannot be delivered for some reason, such as:
Recipient is not a valid or existing email address. Verify that you have the correct email address.
Recipient's mail system rejects the mail for one reason or another
Recipient's mailbox is full
If a message is rejected, you will receive an "undeliverable mail" message, with at least part of the contents of your original message plus some system-generated error messages that provide more information as to why the mail has bounced. Try to correct the problem and re-send the email.
If the mail bounces due to a bad email address, but you are certain that the address is valid, you will have to contact the recipients using other means, and have the recipients resolve the issue with their system administrators.
Contact your system administrator if:
Every mail message you try to send is rejected
Mail bounces with "user unknown" or "bad address" messages for addresses that you used from your corporate address list
You start receiving bounce messages for mail you never sent