First, please ensure the DKIM, SPF, and DMARC records of your sender domain is set up and working properly. You can take the help from this video tutorial for configuring DKIM, SPF and DMARC records from within MailBluster.
Verifying domain ownership helps to increase email deliverability, but there are other steps you need to consider as well.
The second most important is to use Action URL. You can set it from Settings > Custom URL tab. Custom URLs can be used to rename the links that MailBluster uses. For example, web version URL, unsubscribe URL etc. By default, it will use our own URL which is used by other users as well and lacks good reputation. Action URL will help you to resolve this case.
Here is a tutorial for setting up Action URL: https://mailbluster.com/help/brand#brand-how-to-set-action-url-for-your-brand-in-mailbluster
After configuring all the mentioned settings, send a test email to mail-tester.com and check your sending score.
Please note that your emails might land in the spam folder again and there are opportunities to increase your deliverability. This blog might help you: https://mailbluster.com/blog/the-ultimate-guide-on-how-to-prevent-your-email-newsletters-from-going-to-spam