This cron will automatically delete e-mails from the Sent folder older than seven days
find /home/[cpanelusername]/mail/[domain]/[e-mail account]/[folder]/cur -type f -ctime +7 -exec rm -f {}
\;
Replace items in the square brackets with the account / folder details e.g.
find /home/adminletusletuco/mail/admin.letusletu.co.uk/noreply/.Sent/cur -type f -ctime +7 -exec rm -f {} \;
This cron will look in the adminletusletuco cPanel account for the domain name admin.letusletu.co.uk in the noreply@ account and delete all e-mails older than seven days from the Sent folder.