Bubble uses SendGrid unless you specify a different service, and SendGrid mails to Yahoo, Hotmail, Outlook, Live, etc routinely get blocked. So if the emails that are not getting delivered are intended for these recipients, SendGrid is the problem, not Bubble. You’ll need to use a different email service.
I’ll explain more clearly. There is no native email feature in Bubb;e. What looks like a native send email workflow is actually implemented through SendGrid, because Bubble has chosen SendGrid. In other words, unless you actively setup another email service, your emails will be sent through SendGrid. And will get blocked by many recipients.
Because the problem is with SendGrid (search the Bubble forum for “SendGrid” and you will see a lot of information on why that happens [poor reputation management, in a nutshell]), there is nothing that Bubble can do other than replace SendGrid as its default email service.