Consistency and volume
Don't send out emails in random intervals because it's what spammers often do. Instead, use a tool that allows you to send emails in increasing intervals.
Your email recipient list might have address which don't exist, hence they'll bounce. The higher your bounce rate, the lower your domain reputation is so to keep bounced emails low, ensure the veracity of email addresses. Double check spellings and domains as there might be misspells or wrongly-used email punctuations (full stops and hyphens in particular).
What about you? How do you make sure your emails are delivered to your recipients' mailbox, and not the Other/Clutter or Junk/Spam? How do you improve your email deliverability?