Question mark next to email address when receive email sent from Zoho custom domain on personal gmail account

Question mark next to email address when receive email sent from Zoho custom domain on personal gmail account

I've noticed on gmail that most emails have the first letter of the senders email address when they are received.  However, emails received from my zoho custom domain which I paid Zoho a fee to create and use have a big question mark next to them.  Some reading on the web suggests the issue may be related to encryption but I'm pretty unclear of the cause and how to cure it.  I don't know if emails I send from the Zoho custom domain to others are appearing with a question mark but I don't want that to happen.  Can anyone clarify the reason for the problem? And the solution?  Thanks.