There are numerous reasons mail providers place messages into the Spam folder. While there is no guarantee to ensure your messages arrive in the inbox of your recipients, there are however industry standards and best practices you can follow to give your emails the highest likelihood of meeting all mail provider and spam filter expectations.
Encourage your recipients to do certain things can help to support the trust ISPs have for your messages. Check the below with your recipients:
"Add us to your address book!" - Having a recipient add your from address you their address book or trusted senders list can go a long way. More often than not, if one of an ISPs recipients trust a sender, they will be more lenient to similar messages to different recipients!
Star or Mark as important - A simple inbox action like this is just another way your recipients can tell their mail providers that "Hey, I want these messages".
IP Whitelist - Some ISPs or mail admins can add rules to always allow all incoming mail from specific IP addresses, Domain or email! Consider reaching out to our Support Department(firstname.lastname@example.org) and/or ask your reciepient to contact their ISP/hosting compining of problematic mail domains to see if they can white list your domain.
"If you don't receive an email right away, please check your spam folder and mark "not spam"" - Adding this simple sentence to your sign-up form area can solve a lot of potential heartache. If a message you sent ends up in the Spam folder, and the recipient manually goes in and pulls it out, that's fantastic! This not only helps an ISPs incoming mail filters in avoiding false positives, but also improves your standing with that ISP.
I hope this answers your question.