Emails are returned

We have a fortimail installed as a gateway and all external mails are passed through the fortimail.   For last couple of days internal employees are complaining that the mails to external domains i.e gmail and yahoo are returned.   error message : Your message wasn't delivered because the recipient's email provider rejected it. error 2 :  Remote Server returned '< #4.4.7>'   could anyone guide what needs to be done to rectify this on fortimail or is this to do with exchange server we