Unlike the black and white of spam and ham, there are several types of reputation and its all scales of gray.
Much like a small town, the email servers all talk to each other about what mail gets sent a billion times and who is reporting what as spam. Then when they see your mail again they will let the recipient's inbox know it's suspicious and then it gets placed in spam.
But there's a few things you can do that everyone likes (basically, follow good email marketing standards) to ensure you have a good sender reputation while still sending a lot of mail. Which makes sense, right? Doing something well a lot makes you reputable. And the same applies for email.
An email being Ham vs Spam ultimately depends on several types of reputation - each of which involves a lot of moving parts. It also involves how the mailbox receiving your mail is configured to trust email. Which means what one email server sees as Ham is Ham to them and what the other sees as Spam is Spam to them. Even if it’s the same message. Beauty is in the eye of the beholder and all that.
Reputation Goes up when
- People reply to your Email
- People mark your Email as important or at least not-spam
- People open your email several times or forward your email
- Your email address is "warm" (has a recent history of sending good mail)
Reputation goes down when
- People mark you as spam
- People ignore your email
- You send too much mail (More than 200 per day with a Gsuite email address)
- Your email address is "cold" (has no recent history of sending good email)
- Your message includes links to other domains
- Your email bounces (recipient inbox doesn’t exist)
You can check your Sender Score at SenderScore.org. It’s basically just a 1-100 point value that tells you how much the email servers trust email from your domain and ip address. Above 70% is a pass, and above 80 is pretty good. Below 60 is going to hurt.