We strongly recommend using procmail to forward your email. It allows you to filter out detected spam before forwarding messages to another site.
Most new users already have a default /.procmailrc file that will filter out spam and viruses. If you'd like to use procmail to forward your email, you should simply need to append a rule like this one to the end of your /.procmailrc:
# # Forward everything to me at gmail # :0: ! email@example.com