Some letters from Box are not reaching me.

There is a client who has several orders per year. The problem has been recurring for several years: we send an invoice to the post office from Box - it does not arrive. We send it from the mail program - it arrives. The client has Google mail. We have Google mail. The client says he doesn't see it in spam. Most other customers receive their Box bills (If someone doesn't get it and doesn't call us and tell us about it, we won't know). The problem is most likely somewhere on the recipient's side. Either the recipient's email program or Google considers emails sent from Box to be "incorrect" and accepts emails from the same address from the email program. Question: Is there anything we can change in our mail settings to try to solve this problem? (I don't want to dig deep, because such cases are rare)