It started happening to me in back in September. Prior to that date I was able to send emails without problem, all of a sudden I'm above the 10% bounce rate because all of the yahoo emails of my contacts. It does sound like HubSpot is not properly building the email according to Yahoo policies.
But the first point for HubSpot to even look at the issue to lift the suspension is to upgrade to a paid package, which seems to be absurd if they are the cause of the issue.
If I send emails to those same Yahoo addresses without HubSpot (same email content) the email arrives without issues.