undeliverable emails

A financial company has notified me their emails sent to my Xfinity email address are being kicked back as "undeliverable". I had been receiving emails from the company without issue for a long time. My Xfinity email is working fine otherwise, as I'm receiving emails from other companies, including statement alerts, plus publications and individuals. I contacted Comcast Security, but apparently all they did was erase the filters I'd established for spam. It didn't help. I would prefer to continue using my Xfinity email with this company. Are there any thoughts on what the problem could be and how to fix it? Thnx.