Using Discourse as a mailing list is already a power-user type feature, so it’s fair to expect site owners to provide their users with some extra educational material on best practices.
Here’s some docs from Mailchimp:
My open rates seem high.
Here are some common issues that can cause your open rates to be too high.
A spam filter is causing high open rates.
Aggressive spam filters will click links in incoming mail before delivering them to make sure there isn’t any malicious content. Our system automatically tracks clicks as opens, and we have no way of differentiating when a click is from a spam filter. This can sometimes lead to an unusually high number of opens from a single domain. If you notice this on your report, it’s likely the result of a spam filter. We recommend you ask your subscribers to whitelist MailChimp’s servers.
Your subscriber forwarded your email.
When your subscribers forward emails through their email client, any opens for the forwarded email are recorded as additional opens for your subscriber. Encourage subscribers to use the Forward to a Friend link instead to accurately track your opens.
Private email domains tend to have more aggressive firewalls and spam filters than Gmail or other free webmail providers. Sometimes, these network firewalls and filters can block your campaigns.
If subscribers from a certain company or private email domain have trouble receiving your MailChimp campaigns, ask them to whitelist MailChimp as a possible solution.
In this article, you’ll learn about whitelisting.
Maybe it’s too niche for core, but I’m sure a lot of email-centric communities would love to have this as a plugin.