Using a mac alias

Are you sending from Cox email to another domain? If so, what domain? google.com, yahoo.com, etc. 

Also, what are you sending from? Apple Mail? Is that what you mean by a "mac alias"?

Just going by a blind search, error means: "5.1.1 means that the recipient email server believes that the email address does not exist at the destination domain". Could you just have the email address wrong, or something is wrong on their end? Can you send to them from a non-Cox email?

Hello zharry,

Are you able to send to any other email addresses?  Are you sending from a Cox email address and which email program are you using? -Derrick