Exchange smtp error 5.2.3.

Hi i recieve the below error when attempting to send to my exchange server.
'This message is larger than the current system limit or the recipient's mailbox is full. Create a shorter message body or remove attachments and try sending it again.

I've checked the mailbox size, delivery restriction limits in AD and i cannot seem to figure out why this is happening.
Any help would be great.

Thanks
Colm