Hello
Thank you for reaching out in Apple Support Communities. To help troubleshoot the email account with Yahoo in the Mail, we suggest to remove and re-add your Yahoo mail account. This is a step may help resolve your issue. Here's how to perform this step:
Remove your email account and set it up again
1. On your computer, sign in to your email provider’s website. Make sure that all of your email is there or
make sure that your email is saved somewhere other than on your iOS or iPadOS device.
2. On your device, go to Settings > Mail, then tap Accounts.
3. Tap the email account that you want to remove.
4. Tap Delete Account.
5. Add your account again.
If the steps in this article don't help, contact your email service provider for more information.
These steps are outlined in this helpful article that can help: If you can’t send email on your iPhone, iPad, or iPod touch - Apple Support
Kind regards.