The IP would allow you to start building your own reputation for sending emails. There are several things that can be done to help email delivery. The largest factor is your reputation (what percentage of the emails you send are read vs marked as spam).
Also, getting members to whitelist their email with their email service provider (Yahoo, hotmail, gmail, etc) helps. Also, we can setup Sender IP, DKIM, and apply for whitelists with providers that allow it. Most providers require a 3 month history before applying. Return Path (returnpath.com) offers email certification which helps with delivery with many providers. They charge $200-$500 annual fee, plus a setup fee, and require you to have a history of 3 months of emails being sent.
To handle spam we charge email overages and use that to determine a sites usage instead of bandwidth or other methods.
Our email delivery is not perfect. It is typically better than others because we charge for emails,
plus good enough where you can largely develope YOUR OWN reputation with negative impact from us (a more neutral launching pad).
Also, if a bigger issue, we offer dedicated servers with dedicated IPs that we manage NOTE: Additional setup fee's will apply.
Then, you can do a wide range of additional things (like purchasing certification, 100% own reputation, etc).
It's possible to use a separate SMTP server, but the email accounts for members won't work. This won't prevent anything from function (like notifications), just they will no longer have their own email account.