How To Send Mass Emails
The only way to legally send mass emails and not look like a spammer is via an "opt-in" email list. If mass emails are being sent without users opting-in or without providing the ability to "opt-out", you could be violating the CAN-SPAM Act.
Additionally, Office 365, G Suite, and Rackspace email hosting services all have daily email send limits. Exceed these limits and your ability to send email will be blocked for a period of time.
We use MailChimp to send to many recipients. More options are listed here, 7 Best Email Marketing Services for Small Business (2018).