The best practices to send mail merge with Gmail
1) Warm-up your email account
2) Collect responses and reply back
3) Personalize your emails
5) Give your account a profile picture
6) Make sure your contact list is clean
7) Provide a clear way to opt-out
8) Don't send too many emails at once
9) Make sure your messages are authenticated
Why are my emails going to spam with Gmail?
Why Google blocked my emails?
Legal considerations about spam
How to avoid the Gmail spam filters