Using Aliases

I have several aliases set up for my email SEND.  I have added a new one but get an error message of “Mailbox Unavailable”.  Am I missing something in the setup?  It’s been a while since I set up the previous aliases and I cannot find any instructions in the HELP Contents.  Thanks.

I would think that this means that the recipient address in the message you sent is unavailable. Where did this error show up? Can you post a screenshot?