Emails From Contact Not Being Received

Same problem for emails from costco.ca, they do not arrive in any folder. I contacted Costco and they sent a specific email message to my account but nothing arrived. Something in the Yahoo/ATT firewall is eating these messages leaving no trace. They don't even get flagged as spam. They don't arrive in any browser or email app that I am using for this account, so it is not a simple "clear the cache" solution. Needs to be fixed.

I am having the same trouble

@Capricorn58 and @Radion just FYI, I did not mark the response from AT&T as the accepted solution. Either they marked it as accepted or that process is automated if a response is not received. I unmarked it as the accepted solution. Read below.

@ATTHelp - Thank you for your suggestions. One week after my post, I finally received a statement email from that company, but I didn’t change anything that I was doing. I was going to wait until I received one more statement email before I responded to your suggestions, but since other people are also having this issue I decided to respond now. 
I did not clear any cache or cookies because this was happening on my personal and work computers, on multiple browsers, and on two mobile devices. This is not a cache/cookie issue. Their email was not blocked, I already had their email in my contacts list, and there were no filters set.  On my computers, I sign out of webmail daily so that was not the issue. Per my password manager, it would take centuries to hack my passwords, so I doubt that was the issue. I don’t know if the company changed something that they were doing or if AT&T/Yahoo changed something, but I didn’t change anything and I finally received the statement email. So I don’t know what the solution was. Around Jan 8th I expect to see another statement email from them. If I don’t receive that email, I will update this post to let you know that I still have the problem. 

Thank you for all of the follow-up and explanations. This happens at my company from time to time, emails are flagged for the bit bucket by the firewall before they even get on the inside to individual user inboxes (or junk folders). Often, a domain is flagged because of abuse but the net is too wide and it catches legitimate traffic. If Yahoo/ATT were to check their voluminous rules they would likely see that costco.ca is triggering something that kills the email on sight. Once they modify that rule the problem goes away, but the rules are complex and changes are generally made cautiously and, therefore, slowly. I was told by the Costco agent that the full email address is [email scrubbed] if I got it right. Yes, she said the "+" is correct, maybe some ancient Yahoo code cannot handle that!

Thanks @Anikate25 these are all prudent steps. I have not set any filters or diverted any email via rules. I search on "costco" and only US side email comes up. I can try putting the address in my contacts list, but that should only move email from spam to the inbox. It certainly seems like this filter is further up in the network. I find getting the attention of a sentient being at ATT/Yahoo is a challenge, when drilling down the support links it led me to this community blog. They'd rather have the community solve problems than invest in the staff to get it done themselves. Such seems to be the case with most of the big .coms today. My fix when this happened previously is just to change to gmail. I am seriously considering that migration on a wholesale basis. Thanks for the response.

Looking back I see the exact Costco email address in my post was scrubbed but I think it might be instructive just in case an ATT/Yahoo person is monitoring these blogs. Spelling it out the address is "alerts+costco" at "costco dot ca" which hopefully will not get scrubbed. I do not know what possessed them to use the plus sign, but some old code might choke on such a character in an email address.