1.5 billion people use Gmail every month and over 5 million businesses pay Google to use Gmail as part of G Suite. Gmail based addresses probably make up the majority of your email list.
If you use an ESP familiarize yourself with the protocols they use to monitor activity and stay within their guidelines.
However, don’t depend 100% on their screening process. The overall best practice before your emails get canned as spam in Gmail is to manage your list regularly, culling those who never engage. Also, use different strategies for contacts that are new and only somewhat engaged.
When you want to grow your list and bring new contacts online, don’t mail at the same pace as you do for your most active subscribers. Doing so will increase the risk that Google will tag future messages as unwanted. Increasing the sending volume too quickly can result in delivery problems. A good rule of thumb is to increase your volume about 10% weekly. You can increase volume faster but at the same rate if you mail daily vs. weekly.
If you send large volumes of email, Gmail recommends:
Send mail at a consistent rate. Avoid sending mail in bursts.
Start with a low sending volume, and slowly increase the volume over time.
As you increase the sending volume, regularly monitor the sending rate and any responses you get. Look for errors such as low open rates for new segments, bounces (soft and hard), unsubscribes and complaints.
These factors affect how quickly you can increase sending volume:
The amount of email sent: The more email you send, the more slowly you should increase sending volume.
The frequency of sent email: You can increase the sending volume more quickly when you send daily instead of weekly.
Recipient feedback about your messages: Make sure you send only to people who subscribe to your emails and give recipients an option to unsubscribe.