One of our clients has sent messages in his organization, and none of them were received. Then she sent a 'check' message and it arrived together with all the delayed messages.
What can be the cause of it?
Update: It was caused by space shortage on the partition, which in term was caused by Hyper-V bug that shrinks the 'expanding' Virtual HD to the size of it's content. I restarted the server and the VHD returned to it's original size. To prevent similar errors I converted the 'expanding' VHD to 'fixed' VHD.
Everything runs normally now. Thanks.