Several factors decide whether your mail lands in inboxes or spam folders.

• Missing authentication. Without SPF, DKIM, and DMARC records, providers distrust your mail. Set these up (article 86).
• Poor sender reputation. A new domain or one that's sent spam in the past is treated cautiously.
• Spam-like content. Lots of links, all-caps subjects, or "spammy" words trigger filters.
• No unsubscribe option in bulk mail.
• Blacklisting. Your domain or IP may be on a blocklist.

Tip: Setting up SPF, DKIM, and DMARC correctly (article 86) is the single biggest improvement you can make to deliverability.