To mitigate the changes of your posts landing in a spam folder, please review the following recommendations:
1. Make sure your lists are configured to handle DMARC. If you allow AOL and Yahoo users to post to your list, then this is an absolutely requirement. We have two articles on DMARC in our knowledgebase. You can access them here and here.
2. If your users are using Gmail then please have them create a filter via their Gmail account to filter all incoming messages from your list as not spam. It is our opinion that Gmail is way too aggressive in tagging incoming messages as spam.
Settings->Filters->new filter->fill in criteria, probably the mail list "from" ->" Never send it to Spam"
3. Setup a SPF record for your list domain name. If you are using a sub-domain then you will need to setup a SPF record with your DNS provider for your list sub-domain. The SPF record will basically validate all mail coming from your list domain with the our mailman server's IP address. Having a SPF record will improve the deliverability of your list posts. If you are using a domain name and our nameservers, then we automatically setup DMARC and SPF records for you.
4. Have your list members add your list domain to their whitelists whenever they can.
5. Instruct your list members to never report a list post as spam (AOL users are notorius for doing this!). Make sure your list members have clear instructions given to them on how to unsubscribe from your list and make sure you honor all unsubscribe requests.
If you have further questions regarding this, then please feel free to open up a new support ticket.