Ordinarily, when mailstats produces its output, it prints the human-readable names of the delivery agents in its rightmost column:

% mailstats 
Statistics from Sat Jan 1 17:30:02 2000
 M   msgsfr  bytes_from   msgsto    bytes_to  msgsrej msgsdis  Mailer
 0        0          0K      246        685K        0       0  prog
...                                                              
                                                                 here

If you prefer to omit delivery agent names, you can suppress the last column with this -o command-line switch:

% mailstats -o 
Statistics from Sat Jan 1 17:30:02 2000
 M   msgsfr  bytes_from   msgsto    bytes_to  msgsrej msgsdis
 0        0          0K      246        685K        0       0
...                                                             
                                                                 gone

Note that this -o switch can be combined with the -p switch (described next) to also suppress printing the M line in that output.