How do I stop my I Pad deleting e mails?

Can you explain further?

Why do you think it is the iPad deleting the email messages?

Do you see the messages on the iPad for a while before they are deleted?

Do you access the same email account(s) from any other device(s)?

Are the message in question visible on the other device(s)?

What type of email account(s) do you see this on?

If your iCloud account works as you expect, let's focus on your other accounts which are misbehaving.

You say they are IMAP accounts. Is it possible that they were set up for POP access on the iPhone (perhaps a long time ago)? That could explain the behavior you are seeing.

I should also ask whether this iPad is new (to you) and came with this behavior, or whether the behavior was different previously.

Normally, when a device or group of devices use POP to gather email from a mail server, either one of the devices or the server is set to delete mail from the server after a set period of time. That way, the device(s) get a chance to download each message before it is removed from the server. That's fine as long as there are only POP clients using this account. The problem arises when you introduce an IMAP client for the same account without disabling the deletion; the IMAP client just displays what it sees currently on the server.

Is the iPhone 8 accessing this email account via POP or IMAP? If via POP, is it set to delete messages from the server after 24 days?