I am having the exact same problem. Emails sent through the CRM are shown as sent but are actually never received by the recipient. I currently have a Gmail account linked to the CRM.
I've scoured the forums and haven't had any luck with any of the suggestions given so far.
e use the free tools to enter our leads. When I go to send emials through HubSpot the format is completely altered and any/all line breaks are removed.