Dear /v/programming, I hope this is acceptable here. I need your help.

I work at a business that sends regular bulk emails to thousands of people. This isn't spam. They have signed up with us and we need to be able to email them. Recently we have fallen afoul of various spam filter programmes. Turns out some spammers got into the mail relay server, sent out a few batches of spam emails, then vanished. It was enough to get our IP blacklisted in a few places.

I am an IT semi-professional, in that everything I know about how email works I have learned in the past 6 weeks. I have taken us off of a few blacklists. The real IT guys have tidied up a few things, including the way the spammers got in. We are being blocked by "Spam Assassin", which has a long list of rules and I don't know which is relevant to us. I control what's in the body of the email. The IT guys deal with the server. To make matters worse, the server is used by a couple of sister businesses.

To prevent legitimate email looking like spam, what rules should I implement? I am aware of putting the business name and address at the foot of the email and adding an unsubscribe link for non-essential email. I'm currently enforcing these two. What else is important? Assume I have moderate influence in the business, very little outside the business and no budget.

TLDR: How do I stop legitimate business email looking like spam?

Update: Finally managed to get us off a privately-held blacklist. Fuck spammers.