Submissions not received on some email accounts

These are the test results:

From website
https://betterfantasygaming.com

When a submission is sent, we get everything when it is sent from a computer or laptop and tablet. HOWEVER from a CELL PHONE:

We get it on some gmails and mail.com email accounts but not others, but not sure if this matters, the ones that do NOT get received were accounts registered in Europe, and ones that work were from USA.

This is a repeatable and still existing problem.

I even tried forwarding to another email over, but that still doesnt work.

My hosting company had me install an SMPT plugin, and they also adjusted the internal spam filters to make it more forgiving.

Still no.

Solutions?