1. Check that your email follows best practices for sending email, such as using a well-formatted subject line and including both plain-text and html versions of the message.
2. Make sure you are not using any words or phrases that might trigger anti-spam filters.
3. Ensure the sending reputation of your domain is good by running an IP and domain reputation check.
4. Ensure your email server is properly configured and is not set to send a high volume of emails within a short time-frame.
5. Use a professional-looking email address, not a free public email address.
6. Carefully review the email content for any suspicious words or attachments.
7. Ask your recipients to add your email address to their contact list.
8. Whitelist your domain on the recipient mail server and arrange for their IT department to add an SPF record to their domain.
9. Contact the recipient's IT department directly and ask for help troubleshooting the issue.