One person does not get my emails...

A friend of mine almost never receive my gmail emails. Very few of them and we can’t see what those he receive has in common… 
We have different provider.
If I sent the message with my cell phone or send them directly from Gmail website, he ALWAYS receive them
My email address is not on his black list and he does not get them even if he stops his firewall . 
He ask his provider and they say all is ok on their side.  I asked mine, same answer.
The only commun element of the not received email is emClient.
What is the problem
Can you help me to find whatever prevent my emails to get to him? 
It s been like this for a loong time.  If nothing else work, I’ll have to change client.  that would be very sad since emClient is my favorite.

Are the missing messages in his spam folder?