#5.7.1 smtp; 550 5.7.1 Relaying denied> #SMTP#

Hi Folks 

I wonder if there is any Message Labs gurus out there taht can give me some advise:

We recently moved over to 365 email system and ever since any of our previous senders on message labs are getting bounce back messages with an error 

Your message wasn't delivered due to a permission or security issue. It may have been rejected by a moderator, the address may only accept e-mail from certain senders, or another restriction may be preventing delivery.

The #< #5.7.1 smtp; 550 5.7.1 <>... Relaying denied> #SMTP#

any help would be apprecated