I am having this same issue (not to sound like a broken record). I just found this discussion from a search online. I noticed this issue about 2 or 3 days ago. I was getting email from all of my accounts, except for Yahoo. It started working again about 2 hours later, so I assumed it was my imagination. Then, it went down again today. I checked by sending an email to myself, and I am able to send with no problem. Also, I checked on my iPhone, and that seems to be working fine (all new messages and my test message were in my inbox). I also notice that when I try to manually check the mail, "yahoo" is greyed out as if all mail has already been received.
I know there was one opinion in this chain of communication that indicated getting a specific email address for yahoo mail, deleting..., etc. I know that can't be accurate, since I all of us are receiving the email on our other Apple devices. Clearly there is a problem the interaction between the Mail program and the Yahoo Servers; however, it doesn't appear that anyone from Apple has acknowledged that this problem is happening.
Hopefully, I won't get an angry response from the user who indicated we are "griping and moaning." How would Apple ever know we were having issues, if we didn't post them somewhere. I would think that data would be helpful to them for identifying with problems were isolated incidents and which ones were systemic. Also, it is very helpful to those of us out here who are wondering is this experience is happening to others. I found most of these posts very helpful in (at least) validating the problem was outside of my control and preventing me from wasting too much time trying to fix a problem that was not mine to fix.
I guess, in the meantime, I will check my email on my phone, until things start to work again in the Mail program. Regardless, thank you to everyone who posted here. It was very helpful to know that this issue isn't just isolated to my computer. 🙂