Are you currently using the HubSpot extension in Outlook 365? If so, if you send the email from your inbox tracking and logging to HubSpot does your contact receives the email? ( More information here)
When you send the email from HubSpot are you able to see the email in your send folder?
Does the email is delivered to the spam/trash folder? What's the status of the email in HubSpot? Are you seeing any errors?
The more information, screenshots, and details you can provide, the better I can advise on the next steps.