First, please ensure the DKIM, SPF, and DMARC records of your sender domain are set up and working properly. You can refer to this video tutorial for configuring DKIM, SPF, and DMARC records 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 step is to use an Action URL. You can set it from Settings > Custom URL tab. Custom URLs can be used to rename the links that MailBluster uses, such as the web version URL and unsubscribe URL. By default, MailBluster uses a shared URL, which may lack a good reputation. Setting up an Action URL will help resolve this issue.
Here is a tutorial for setting up an Action URL: 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 still land in the spam folder, but there are opportunities to improve your deliverability. This blog might help you: The Ultimate Guide on How to Prevent Your Email Newsletters from Going to Spam.