Hi Sophie,

Most iOS Mail app issues are caused by invalid settings or if there's something wrong with the Yahoo Mail account that you're trying to add.

1. Check if your account works outside of the app

1. From a computer, open a web browser.
2. Log in to your Yahoo Mail account.
3. Compose a new email and add your own address in the "To" field.
4. Send the email and see if it arrives within 5 minutes.

If any issues occur, you'll need to address those before the account will work in an app.

2. Update your operating system
Apple releases updates to their operating system any time they identify a fix to an issue. Make sure you're using the latest version of the iOS software.

3. Remove and re-add Yahoo Mail to iOS Mail
Settings that connect Yahoo Mail to the app can be incorrect or corrupt.
Remove your Yahoo Mail account from iOS Mail.

1. Open the iOS Settings app.
2. Tap Accounts & Passwords.
3. Tap your Yahoo Mail account.
4. Tap Delete Account.

Re-add Yahoo Mail to iOS Mail.
This option is available to newer versions of the OS.

1. Open the iOS Settings app.
2. Tap Accounts & Passwords.
3. Tap Add Account.
4. Tap Yahoo.
5. Enter your Yahoo Mail address and password, then tap Sign in.
6. Optionally, disable aspects of Yahoo Mail from syncing.
7. Tap Save.

Use this option for older versions of the OS, which do not support the automated setup.

1. Open the iOS Settings app.
2. Tap Accounts & Passwords (or Mail, Contacts, Calendar on older devices).
3. Tap Add Account.
4. Tap Other.
5. Enter your full email address and password, then tap Next.
6. Enter our IMAP settings for incoming and outgoing servers.
7. Enter your email address for the user name, then tap Next.
8. Make sure "Use SSL" is on and "Server Port" is 587, then tap Done.

4. Use the Yahoo Mail app

If you're still unable to connect to the iOS Mail app, there might be something wrong with the device. You can still access Yahoo Mail by downloading the Yahoo Mail app.

I hope this helps.

Regards,
Lokesh