Emails from financial institution not received

A local company recently sent two emails to my Xfinity email address which I never received.  I know the address was correct and they did not receive an undeliverable message.  After I spoke with them, they sent it to my Gmail address and I got it almost immediately.  I have just received the second letter from my financial services company (one of the largest in the US) saying an email to the same Xfinity address came back as undeliverable yet I have received multiple emails to the same address from this same company in the past week.  I use Outlook as my email client and it's set up as Imap.  I check my junk/spam folder every day so I know the missing emails are not going there.  

Is there any way to determine why these emails have not been delivered?  Or whether the problem was at the sender's end rather than Xfinity?