[email] How do I purge a linux mail box with huge number of emails?

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.