that is strange
looks like a version specific option
[david@www david]$ mail -H
mail: invalid option -- H
Usage: mail [-iInv] [-s subject] [-c cc-addr] [-b bcc-addr] to-addr ...
[- sendmail-options ...]
mail [-iInNv] -f [name]
mail [-iInNv] [-u user]
[david@www david]$ mail -i
Mail version 8.1 6/6/93. Type ? for help.
"/var/spool/mail/david": 4 messages 3 unread
>U 1 forum@linuxquestions Sat Apr 12 17:12 44/1692 "Reply to post 'noob question on slackware'"
U 2 forum@linuxquestions Sat Apr 12 17:33 44/1715 "Reply to post 'US Robotics Serial modem problem'"
3 forum@linuxquestions Sat Apr 12 17:34 45/1747 "Reply to post 'read mail headers from command line'"
U 4 forum@linuxquestions Sat Apr 12 23:04 44/1739 "Reply to post 'Partition Help'"
&
david@zeus:~$ mail -
mailx version nail 10.4 1/19/03. Type ? for help.
david@zeus:~$ mail -H
>U 1
david@zeus.domain Sat Apr 12 23:05 36/1412
N 2
david@domain.com Sat Apr 12 23:15 13/662
-i seems to work