One particular email did not get forwarded to my yahoo account

I forward my emails to my yahoo account. I have received and sent many emails to customers fine. However, I didn't receive one particular customer's response to an email. She said she sent it yesterday, but I don't see it. I've checked/searched (including my spam folder) and don't see it. I finally logged into Roundcube and I see it there. I tried calling tech support but was told that it was an issue on the customer's end. This doesn't make sense to me since obviously she received my email and sent a response, and it's in Roundcube - it just wasn't forwarded to my yahoo account. The only thing I can think of is that her email is a q.com email - would that be an issue? Any one encountered something like this? I really don't like the fact that I may be missing customer emails.