Is there some log or some manner in which I can trace why specific users have issues receiving verification email?
They say they have checked their spam folders - not there.
Today I have this issue with a user with an email address at free.fr
I did a bit of a search and there are some hits that perhaps the mail servers at free.fr may return 5xx return codes for malformed email or somesuch. I'd like to see if I'm getting anything back.
OK, not to worry. We found the issue:
free.fr is rejecting the email as spam.
I'm not about to jump through whatever hoops they have to get whitelisted on their side.
(https://i.imgur.com/jOxJLPy.png)
Make sure you're sending emails via SMTP if you aren't already. That will greatly reduce the likelihood of it being flagged as spam.
And if your using your own hosted email, ie not a third party email such as Gmail make sure you have correctly set up SPF / DMARC/ DKIM records or it won't just be these few receiving mail servers you have marking you as spam that you have issues with, it'll be a lot more. You might want to check your domain against internet blacklists.