Spam placement is usually caused by domain authentication, sender reputation or list quality rather than by Odoo itself. Work through the technical points first, then the content.
- Check that the email from address on the mailing uses your own domain, and that your domain has correct SPF, DKIM and DMARC records for the server that actually sends the emails.
- In Email Marketing > Configuration > Settings, review whether you use a dedicated outgoing mail server for mass mailings; if you do, make sure that server is authorised in your SPF and DKIM setup.
- Clean the list: remove addresses that bounced, and use the blacklist so they are excluded automatically from future sends.
- Start with smaller segments of your most engaged customers, then gradually increase volume.
- Review the content: avoid a single large image with little text, keep a clear unsubscribe link and a plain, honest subject line.
After these changes, compare the bounce and open statistics on each mailing. Steady improvement over a few sends usually shows that your reputation is recovering.