It's definitely a big issue if users are unable to confirm their email. It looks like it's happening to majority of new users. I'm not sure if some are using the social logins (twitter, facebook, google) and that's working a different way, but I do know that when I try to signup with an email/password, then it's not sending the verification letter and I have to continually click the resend link.
I think I may have fixed my issue. I'm using the SMTP mailer module to send emails through Sendgrid. It looks like the credentials stopped working in my Una site. I created a new API at send grid and added it to SMTP form in my site.
Going to perform further testing to see if this is no longer an issue.