I wanted to refer you to this online resource regarding this soft bounce message.
Soft bounces indicate a temporary problem with the recipient's server, or another temporary technical problem. Contacts who soft bounce are eligible to receive future emails from you.
I would recommend taking a look through the referenced document and checking your domain on MXToolbox.
After you have gathered the information on MXToolbox, I would suggest connecting with HubSpot Technical Support, as Support is included in your subscription. They will be able to provide real-time assistance and have access to internal tools to assist with this matter.
When you contact them, I would recommend including the bounce message, a link to the affected email and the MXToolbox results.