Summary: email sent by users with mailboxes held by yahoo may have their email disappear.
This may be mail to a mail list (such as this one) their mail list postings may not be received by other yahoo users and also gmail users & a few others.
Yahoo made changes without warning and have caused a huge on-line discussion by mailing list operators.
Yahoo broke email lists.
This is because at the start of the week Yahoo changed their DMARC settings to ask servers that receive a message with a yahoo.com email address in the From: line to reject it if it isn't properly signed by yahoo, which all messages they send will be.
Basically, Yahoo has said that it users are not supposed to use any mailing list configured in the manner that they are often configured in.
There are several possible workarounds, but none of them satisfactory. Discussion continues.
See: