Just forwarded a email to email@example.com and it has come back as undeliverable -
Delivery has failed to these recipients or distribution lists:
firstname.lastname@example.org The recipient's mailbox is full and can't accept messages now. Microsoft Exchange will not try to redeliver this message for you. Please try resending this message later, or contact the recipient directly.
Thanks for letting us know, I'm sorry no one got back to you sooner. I've just sent a test email and it's went through without any problems. I presume the mailbox manager has made some room since Thursday.