The files in clientmqueue and mqueue are named with the following namin convention
first letter = type and status
second letter = "f"
rest is the sendmail queue ID that you see in the logs and mailq output.
The first letter is usually one of :
d : the body of the mail
q : routing information and headers when the message is normal
Q : rename of the "q" file when the message is abandoned for some reason
h : rename of the "q" file when the message is held (quarantined by a milter)
t : temporary file
x : transcript of delivery attempts