[Mailman-Users] Preventing from getting on spam list

[Hung Phan]

Hello,

We are a school district which sends out legitimate messages about school information to parents but last week, we were tagged by spamrats as spammer. An important message did not reach all the subscribers because the local ISP which hosts many of our parents' emails, subscribes to spamrats as one of the anti-spam source. 

What measures can we take to prevent us from being tagged as spammer again? All our lists are always set to not allow non-members post. Is it even worth $102 annually to be on whitelisted.org? I believe whitelisted.org will drop us immediately out of the whitelist as soon as we somehow trigger the spamrats again. 

Thank you very much for advice on this matter :-)