Emails not received by people who try to sign up

I have received complains from some people who tried to sign up for a website, but never received the activation emails.
My first thought was that they typed the emails wrong. But checking on phpMyAdmin on signups table, I saw the correct emails.

I have no idea why this is happening. Could be a filter (I don't know, like an anti-spam filter) from their email provider? None of those emails are not the usual gmail, yahoo, outlook, etc.

Did anyone encountered this before?

I'm just looking for solutions... I was thinking of trying a smtp plugin on the main website... or anything else...

Thank you