Mail App on iPhone is Deleting Emails Immediately when received from Yahoo Inbox

Probably it is the way the yahoo mail has been configuredred by you.

If you added IMAP email accounts, the iOS mail app does add IMAP by default.

When you set up an email address in an email client, you will need to decide if you want to set it up as POP3 or IMAP. Both of them are ways to connect to the mail server so you can read your emails through an email client.

• IMAP is short for Internet Message Access Protocol. With IMAP, the message does not remain on the local device, such as a computer, it remains on the server.
• POP3 is short for Post Office Protocol. With POP3 mail, it will connect and attempt to keep the mail located on the local device (computer or mobile).

Pop3 vs. IMAP: Which is better?

• IMAP is better if you are going to be accessing your email from multiple devices, such as a work computer and a smart phone.
• POP3 works better if you are only using one device, but have a very large number of emails. It is also better if you have a poor internet connection and need to access your emails offline. For most people, IMAP will suit their needs better.

Note: If emails start disappearing from your inbox without you deleting them, it almost always means one of your devices is on POP3. Backing up the emails and then resetting up the email account as IMAP will fix the problem.