Gmail published the guidelines below in their Help Center to help you understand how your IP or domain reputation impacts deliverability. The better your reputation, the more likely your email reaches the inbox.
- Bad: A history of sending a high volume of spam. Mail coming from this entity will almost always be rejected at connection time or marked as spam.
- Low: Known to send a considerable volume of spam regularly, and mail from this sender will likely be marked as spam.
- Medium/Fair: Known to send good mail, but has occasionally sent a low volume of spam. Most of the email from this entity will have a fair deliverability rate, except when there’s a notable increase in spam levels.
- High: Has a good track record of a very low spam rate, and complies with Gmail's sender guidelines. Mail will rarely be marked by the spam filter.
Source: Gmail Help Center