Inboxing with Email Marketing

I have a large number of email subscribers that use several different email services such as Outlook, Gmail, and Yahoo. My question is simple: How do I ensure I reach the inbox and avoid the spam folder when sending these contacts marketing messages? I know that there are several variables that determine whether a message reaches the inbox such as sending IP, From name / Subject line, and email content. I was testing my emails on myself to make sure they were inboxing and the emails that were inboxing earlier this week are now getting sent to the spam folder. I have tried using Mailchimp and Elastic email. This is confusing to me because my emails are very simple and text based. Does anyone know any and tried and true methods to inbox with these major email providers? Thanks.

EDIT: I checked the message details of the emails that was sent to spam and the one the was inboxed. The only difference was that the spam email had SpamDiagnosticOutput: 1:6 therefore was sent to spam and the inboxed email had SpamDiagnosticOutput: 1:5. This is strange to me because the two emails are identical. I am unfamiliar with these terms and would be interested to know more if anyone has knowledge on this.