My application's email messages aren't being delivered

[eluser]KeyStroke[/eluser]
Thanks man! it worked!!

I think I've assumed that this library just wraps up PHP's mail function, but it doesn't.

Shall I use it as my main mailing method for a e-mail hundreds of e-mails a day (mostly notifications for users), or rely on something else for that?

Also, any tips on how to avoid spam filters or being considered as spam? because, now for instance, Outlook 2007 considers the e-mail I send through my app as spam.