My Gmail not reaching to the Organization's Outlook

Hi, 

I am sending gmail to my ex employer at the mail id indicated by them. The response which I am getting from Postmaster is that 'The recipient weren't found at XXXX address'. I am sending mail to 'A' and the message is indicating that mails are not able to reach 'B' & 'C' because 'The recipient weren't found at XXXX address'. There is no response from 'A' either. 

And, of late, they are claiming that no mail is received from my end. 

Please suggest what is happening.