Implementing good emailing practices is the most essential part if you want to improve your deliverability.
Here are some tips and hints, to easily improve your email security, to reduce risk of being blacklisted, to avoid being reported as spam by your recipient or to prevent spam filters triggering. As a result, having hygienic sending practices will improve your deliverability as a whole and help you to stay away from spam folders.
Clean your mail list to avoid bounce. Don't use bought mail lists.
Set up authentication protocols (SPF, DKIM and DMARC).
Monitor blacklisting/greylisting.
Send relevant content to avoid being reported as spam by your recipients.
Use List-Unsubscribe Header to make things simple for your subscribers.
Take care of URLs in emails (avoid broken links and shortened URLs).
Avoid using shared-IP to fully control your sender reputation.
Warm-up your email address and domain before mass sending.
Keep steady sending volume, avoid huge gaps in email activity.
Avoid using spammy keywords in subject line and body content.
Segment your mail list to make sure that your content is adapted to recipients.