There are a number of factors that could be affecting your email delivery. The most common reason is if bulk emails are received and left unopened or unread then providers normally identify this as spam.

Other reasons could include:

• Low Engagement Rates: If recipients rarely open or interact with your emails, it signals to Email providers that your content isn't valuable to the user and moves to Junk/Spam. 
  

• High use of Images/Attachments: Emails with a high number of images or attachments compared to text content can also trigger spam filters with some providers. The same can happen if there are a high number of hyperlinks. 
  

• Spam-Like Language: Excessive use of "click here," "free," "limited time offer," or similar phrases.
  

• Technical reasons like invalid SPF, DKIM, or DMARC Records in your DNS records: These email authentication mechanisms ensure that the email is genuinely from your domain. If they're misconfigured or missing, the provider might mark the email as spam. We have guidance on including SPF records in your domains here(Step 2).
  

• High Bounce Rate: Sending emails to invalid addresses can hurt your reputation.
  

• Recipient Personal Filters: Customised spam filters or blocklists set by the recipient.
  

• User-Specific Spam Filtering: For example Gmail heavily relies on individual user behaviour to determine spam (e.g., previous engagement, marking as spam).
  

Mail providers (like Gmail, Yahoo, Outlook, etc.) often implement specific rules and filters beyond general best practices. For example, Outlook recently implement new security measures in a recent update and we BT Internet mail servers have far stricter rules when it comes to bulk mailing compared to some of the other providers. 

Without looking at emails on a case by case basis it's near impossible to determine a cause once it’s left our mail server.