This is driving me nuts!
I can send and receive email from my own and any other domain from a client such as Thunderbird or Outlook.
I can receive and sent from my own domain on the web client. I also can receive from anydomain on the web client. But I cannot send to domains other than my own from the web client.
Error message is Message not sent; one or more addresses were not accepted. Rejected address firstname.lastname@example.org