My emails are being marked as spam

[Hunter Lyons]

I have sent various test emails to a number of my personal / work emails, and they are always marked as spam by Gmail.

A number of people who are beta testing my site have confirmed similar experiences.

We've all gone through many times and marked them as not spam, but they still go to spam.

How can I ensure my emails are not counted as spam by Gmail and that they go into people's inboxes properly?

I understand that I can add the account to my personal Gmail contacts to fix it for just me, but I am more concerned with end users not getting email.

[Philooo]

IP reputation ? 
SPF recond on DNS ?
It can have many and many reasons ...

[opentype]