Do you know what’s the share of customers affected? If it’s less than 5-10% or only specific domains then this could be considered acceptable. Very few email marketing platforms that use shared email addresses successfully deliver more than 90% of emails (see here for stats).
If this affects only a subset of customers with a particular email domain or email service provider, then emails are being blocked or marked junk due to local settings or IP being blocklisted. But I hear you that asking your customers/prospects to do extra work to receive your emails is a tall order.
Have you tried asking Hubspot to move your account to a different IP (that perhaps won’t be blocked for these accounts)?
Alternatively, there’s the cumbersome setup of using another email tool in parallel for the affected domains/contacts. Many email tools have good integrations to Hubspot CRM, so this wouldn’t be a completely disjointed experience.