Emails from specific address not getting to me and mine not getting to them

A customer I have emailed for years just informed me that they have not been receiving the monthly email.  My system shows it going out along with a number of others and the others are receiving their email and all emails show in my sent folders with none reporting being returned to me.  
Also I have not received their emails - they have sent a number of emails to alert me of the situation which I did not receive - they sent an email to another ouffice which emailed me about the problem.

I have checked all  my spam and trash folders and there is no evidence of any emails from or to them and all other emails seem to be working fine.
I use Thunderbird to download from Xfinity and all is working fine as far as I can tell.