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.

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.

Warm-up your inbox and domain before mass sending.