Sending Email with PHP and ensuring it doesn't get blocked

I want to send an HTML email with PHP. I was wondering what headers should be used in the email to ensure it doesn't get blocked or end up in SPAM folders.

The email contents will contain CSS and HTML. The reply-to field will be different than the sender's ( will this affect anything? )

EDIT: I don't know why my question was voted down. Anyway, I'm not looking to email SPAM or mailing lists. The reason why I want the reply-to field to be different than the sender is because I'm creating a web app where a user manages jobs from various clients. Whenever a job is updated or created, the app will automatically send an email regarding job updates to the client. The client can then reply to the email to contact the user - that's why I need the reply-to field to be the user's email address.

Solution

There are many different factors which take into account whether an email is blocked or not, headers play a part but also to consider are:

Consistent number of emails being sent day to day (100,000 once a month will damage your reputation more than 100,000 each day)
If you frequently email to bouncing addresses
Whether you are on the recipient's safe sender's list
The email provider you are sending to
Various DNS settings such as are you sending email from the domain your IP is registered to
Whether you frequently email invalid addresses
Whether you have been blacklisted by one of the blacklist companies
Various other email provider mechanism such as yahoo's domain key
The content of the email, avoid phrases like FREE, MONEY and LUCKY
Whether you include an unsubscribe link ( you MUST MUST provide an easy way to opt out of emails)

For one off emails you are often ok, if you plan to use your application to send frequent mailing lists it may be worth enlisting a third party company specialising in bulk email.