Site members sending emails avoiding spam blockers

We have a site in which members can send emails to each other by groups… so anywhere from 20 to a few hundred emails can go out.

Problem is that various servers are tossing mail into spam blockers, even though our emails are legit and all. Perhaps one reason is that the email may look like spam (From address isn’t coming from a server that has that domain… or other reasons that I don’t know of?)

I’m looking to become educated on what we can do to satisfy all the authentication and all found in the raw headers… how to make our emails where they don’t look like spam, etc.

Can someone give me links or point to a consolidation of information. For example, companies like constantcontact.com must go through rigorous steps to help make sure their emails don’t appear to be spam. I want to know much about what they must do to accomplish this.

Would really appreciate some help on this. Too many of our emails go into gmail spam blocker, yahoo spam blocker, etc.

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