To actually put whatever is in the email into the .html, you could
just put one or two lines in your .forward or .procmailrc file and then
send to yourftpusername@yourserver.com. That would have security
issues, though, so instead we'll pipe it through a program which
checks the input. In .procmailrc:

Code:
:0 H b:
* Subject: update page
| /home/user/bin/email2html.pl

In /home/user/bin/email2html.pl:
Code:
#!/usr/bin/perl

my $htmlpage = '../domain/public_html/blah.html';
open HTML ">$htmlpage" or die "could not open '$htmlpage': $!"
while (my $line = <STDIN>) {
    # Do whatever checks on the input.  Allow ONLY legal characters, disallow 
    # anything else.
    print HTML $line;
}
close HTML;
The above code is off the top of my head, untested, but it's so simple
a task that it shouldn't have any major errors. We're just reading from
standard input and writing to a file.