Just use:
mail
d 1-15
quit
Which will delete all messages between number 1 and 15. to delete all, use the d *
.
I just used this myself on ubuntu 12.04.4, and it worked like a charm.
For example:
eric@dev ~ $ mail
Heirloom Mail version 12.4 7/29/08. Type ? for help.
"/var/spool/mail/eric": 2 messages 2 new
>N 1 Cron Daemon Tue Jul 29 17:43 23/1016 "Cron <eric@ip-10-0-1-51> /usr/bin/php /var/www/sandbox/eric/c"
N 2 Cron Daemon Tue Jul 29 17:44 23/1016 "Cron <eric@ip-10-0-1-51> /usr/bin/php /var/www/sandbox/eric/c"
& d *
& quit
Then check your mail again:
eric@dev ~ $ mail
No mail for eric
eric@dev ~ $
What is tripping you up is you are using x
or exit
to quit which rolls back the changes during that session.