Out of control mailbox

[Joe]

Hey all,

I have a user with a huge mailbox that is borking things up here. I was
going to drop all mail older than the 19th to reduce the filesize. I had
planned to do this by opening it in vim, finding the first line with
"Jul 18" and remove from the line before it to line 0. Problem is, it's
a 160+ MB file and vim doesn't handle that too well. I know which line
number I want to delete to though. So here's the question....

How can I remove from line 0 to line 1000000 (as an example) from the
file? Or, how can I export from line 1000001 to the end in a new file
(which I could then replace the current mailbox file with)? I have no
idea how to accomplish either.

Thanks in advance for any help!

-Joe