Intermittent Yahoo Mail communications (Mail Sync)
I finally poked Google the right way, and found reports that others have encountered the same intermittent mail-sync issue between the native iPhone Mail Application, and Yahoo Mail accounts.
I have encountered this issue intermittently >for years<, across at least 3 generations of iPhones.
It doesn't matter if I'm on Cel Data, or a good local WiFi, such as my home FiOS connection.
I maintain 3 different Yahoo mail accounts, and between all of them, 98% of the time there is no issue with any of them. Then for some reason, one of the accounts (randomly), when it cycles to Checking For Mail, one of the accounts errors out, and only a random amount of time passing clears it up.
Typical: Last Updated time is displayed (bottom), and it is some time in the past hour or two. As I write this, my one account last sync'd @ 7:30pm, and it's almost 9pm. It reports Account Error [an Account Name], and the Details reports "Cannot Get Mail, The connection to the server failed" But of course, the other 2 Yaho accounts, talking to the SAME Yahoo mail server, are just fine. If I try to do anything that requires a mail server query, it will fail every time - only leaving it alone for awhile lets it fix itself.
This never happens with the two other GMail accounts I also have set up on the phone. Ditto for the iCloud mail (which I rarely have any activity in).
On my PC, I periodically use a similar (communication method) Thunderbird mail client, with two of the Yahoo accounts. It's SMTP/iMAP4 communications/sync process are flawless - even while an error is present on the phone for one of them.
As a 30+ year IT professional having administrated/managed several mail server platforms, including user POP3/IMAP4/SMTP experiences, I suspect there is either an undetected flaw in the iPhone mail client, or (more likely) there is something that the iPhone client is doing which triggers Yahoo's anti-abuse/anti-spam mechanisms, and the login is (probably) being denied for a period of time.
The problem of course will be to reproduce it, but with an appropriate sniffer tool watching along the route of communications (which is using secure protocols now).
Yahoo has no incentive to help - they will tell you to use their app so as to flash ads in your face - no way.
iPhone SE (2nd generation)