Apologies for the delay in responding to your inquiry.
If you're still experiencing this issue, can you please share more details such as what type of email you sent (1:1 emails or Marketing emails), screenshots where tells you the email weren't delivered etc?
The more screenshots, details and information you can share the better the Community can assist! I would also recommend connecting with HubSpot Technical Support regarding this matter, as they have the tools and resources to investigate this matter.