BES to BIS - deleting messages

I just switched the company I work for.
Used to be: BES with a Curve - Verizon.

Now: BIS with a Tour - Verizon. (New company doesn't have a BES). They use Outlook ... and I've established my company e-mail address and passwork as an e-mail account on my Tour.

I need help understanding BIS: If I delete messages on my company e-mail or my att-webmail...does that mean that they WON't delete on the handheld? That means I'll need to manually delete hundreds on messages every day on the handheld???? Is the difference between BIS and BES (aside from the security issues) that a BIS doesn't synch in the same way?

What don't I understand here?