Email Certificate Invalid

Hi all;

On both my Macs (10.10.5) I haven't been able retrieve emails from one of my accounts. I get a message that the server's certificate is invalid. I can get emails from that account on my iPhone.

Several days prior I had reset my permissions with Disk Utility.

I have tried to contact my email host, register.com, but have not heard back. I don't know who .hostingplatform.com is.

What is happening?