Hi Jason,
Are you sending using your own mail server or a 3rd party email newsletter service?
Have you checked your outgoing mail logs for signs of abuse?
What is your outgoing IP address? Check your
sender score.
Check some full email headers to verify you are really sending email from the expected IP address and that is has matching forward & reverse DNS.
Do the emails have attachments? Try removing any links and sending a test message.
Do you make it easy to unsubscribe? Maybe too many users reported your email as spam.