Many iPhone 6s/7 users including iPhone 8 and iPhone X users are facing issue with iPhone mail app that their configured Yahoo email is not working properly. Yahoo mail in mail app are not updating in iOS 11. It seems that mail is not getting fetched properly from Yahoo server. But if you go to desktop version there is new email while iPhone not shows up latest emails. Even when tried setting up my yahoo accounts in outlook app and it works flawlessly so this is clearly a mail app issue.
After upgrading to iOS 11, iPhone intermittently receives Yahoo Mail in the app. It typically updates no more than once or twice a day even it is set to fresh in few minutes. The iPhone says it is downloading eg 15 messages and then nothing loads in yahoo mails. The issue is not even reported with latest Apple flagship iPhone X with iOS11. Even on the latest iOS 11.2.5 yahoo emails are not updating.
Adding and removing the email account does not help this issue at all. To fix the issue even hard closing the e-mail app and uninstalled and reinstalled the email account does not seems to help.
How to Fix iPhone X/8/7/6s Yahoo emails not updating in iOS 11:
Fix 1: Update SMTP Settings
The issue was reported with gmail users as well in older versions of iOS, so it is recommended that you change the SMTP settings for yahoo email as well:
Goto->Setting->Accounts &password->choose yahoo mail—>choose account—>choose Outgoing server SMTP—>Choose Add Server—>host Name: smtp.mail.yahoo.com, and then your email address and your emails password, tap SAVE, then your email will update properly.
Fix 2: Check with yahoo IP address
In above Fix, instead of putting "imap.mail.yahoo.com" on the imap line, put yahoo IP address, 18.104.22.168, may help to update normally.
Fix 3: Use Yahooo Mail App
In case you can’t wait till Apple fix for the issue, you can Shifting to using Yahooo Mail App. It is not very user friendly and lacks features - Single landing page for all accounts, calendar sync- just to name a couple of obvious ones. but if it works better on the core basic feature - deliver email reliably/timely.
Hopefully any of above fix helps you to fix the issue. In case none of them helped you, please consider updating to latest iOS released by Apple.