Strange things are happening.
I have several e-mail accounts that forward to my Yahoo! account. I've never had a problem with this before.
Recently, an e-mail from one user was not correctly forwarded to my yahoo! account. I got this error message
Then there was a lot more stuff after that with daemon messages.
I've deleted my e-mail account name from the above picture.
Other e-mails forward to my Yahoo! account from my microsoft online account are working fine.
Curiously, Microsoft Online is also not accepting e-mails from this one user. This user is also a Yahoo! user.
Then I decided to go to online help for e-mail. I posted my question.
A couple of people posted phone numbers purported to be numbers of Yahoo! itself. One of these numbers was from a user called "Yahoo Support." This number was 866-562-7219
I called that number. The person there said my account had been hacked by someone in Phoenix and that I would have to pay money to get that fixed. I became suspicious, because I have no other indication that my account has been hacked. No suspicious e-mails have gone out. I asked how I knew he was really from Yahoo! Then he said it was up to me and hung up on me.
Then I went back to online help and both of the messages purporting to offer me Yahoo! support phone numbers to call to get my problem fixed had vanished.
This is all very unsettling.
Addendum: when this same user writes to me using his gmail account, there is no problem