Sendmail inbox file splitting

[velda]

Dear all,
I have RHEL 4U5 (old, I know, but it works), and on it Sendmail-8.13.1-3.2.el4 .
File size limit (on OS level) is 2G, so when someone's /var/spool/mail/$USER file nears it, I have a problem (it luckily happens rarely). So if user needs only some of the mails from a 2G file, how do I split it up?
I have tried Google, and found nothing Sendmail-specific (all I found is the usual file splitting, and that is not needed here).
Help?