Error Message after sending email

When sending out emails, I am getting an error message that says:
An error ocurred
Cannot send message (Reciepeint address “al” not accepted with reason:

I receive the message upon each and every email I send out. The reciepient “al” is obviously inserted from unknown source. It does not have a valid email address but gets automatically inserted in the CC are. What can I do to resolve this issue? Anyone who had the same problem?

Hi Apollo,
I may have a solution to your issue.
I had same problem due to inserting incomplete address when composing mail and then try sending it.
After that, the wrong address kept popping up whenever I started entering an address with the same initial letters.

To remove the wrong entries from the recipients, just go to Settings/Send.
You will find a button “Clear Recipient History” tha will remove all manually entered addresses that are not in your Contacts folder.
Hope this works for you too.