mailq displays a list of all messages that have not been delivered yet. The display contains: the queue ID; sending user; the size of the message and the time the message was created; the return address, and a list of the message's recipients. If the message was already delivered or failed for some recipients, this will be indicated as well.
The optional -sort sorts the list according to the messages' timestamps, oldest messages first. This may take a while if the mail queue is very large.
Only the superuser can view the contents of the entire mail queue. Everyone else can only see their own messages.