“Mail delivery failed: returning message to sender”

I have added a POP mail to my outlook id. Also, I have added the same email id to my  outlook id as a send only account and using that email id as a default for sending the mails as I have been using the same since long. It has been smooth and undisturbed for a while and now I am facing immense problem with it. by getting "Mail delivery failed: returning message to sender" message to few of my most important clients, and I am forced to send email from other mail ids. Kindly revert back to me regarding this as I am using this mail for my business.