Maybe check if your /var has enough space too. Mails are held in /var/spool/mail/<username> .
You could also try to expand /tmp and after you cleaned up your mails, shrink it, if the space is scarce on it's rootvg.
Either you have a typo missing the l (L) in mail, or you have most probably a different, similar sounding directory in /var where your mail files are being spooled in. For the second case, you have to use your wits.
I have no AIX 6 at hand, but in 5.3 and 7.1 it is /var/spool/mail .
The "mail" command creates workfiles in the directory /tmp . If your mail file is very large (in you case say 50 Mb) there will not be enough space in /tmp to carry out the mail file edit with the "mail" command.
As advised earlier, first look at a directory listing of /var/spool/mail and see how big the mail file(s) are.
If you are in a hurry and have somewhere else with more free space you could try "mailx" having set a new value for $TMPDIR (see "man mailx").
On my system it would take under a year for root mail to reach 100 Mb . We use a cron to archive root mail daily at 12:00 and just keep 2 weeks worth of day files. It's too much labour to delete mail with the "mail" command.