I'm doing a bit of research for work regarding SMTP servers. Basically, our company is bringing some work in-house that we've been doing remotely through other companies. We have the need to work with mailing lists, but I'm not sure how exactly to handle the great number of emails that will be sent out. From what I'm reading, many applications boast an "internal smtp, so you don't use your ISPs," but that just confuses me.
What exactly is an SMTP server, and how does it bypass my ISP? I can't seen to fathom how anything could make it from my network, to the recipients without taxing my ISP's servers. Can somebody shed some light on this?