Header recommendations for mail function

I have opted to create my own mailing list system for my site instead of using an existing software (mainly so I can take advantage of the task crons I've already created that can send out the mail periodly).

Now, I have my system working as is. Since my site's mailing list is opt-in only (users HAVE to sign up on the site to get the newsletter) I want to do as much as possible to avoid having my newsletter wind up classified as spam.

So, I am looking for recommendations on pretty much anything I can do to the header info I send with the mail() function that increase my chances of not being classified as spam, even though the newsletter is purely informative... not a salescopy or advertisement.
MaybeItsJeremyAsked:
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MaybeItsJeremyAuthor Commented:
Thanks! PHPMailer will do. :)
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