How to enable junk mail filtering on iPhone

Use Mail Privacy Protection:

1. Go to Settings on your iPhone.
2. Scroll down and tap Mail.
3. Tap Privacy Protection.
4. Ensure Protect Mail Activity is turned on. This prevents senders from using remote content (like tracking pixels) to see if and when you've opened an email, which can reduce the frequency of spam you receive.

Whitelist Senders: Adding a sender to your Contacts can help ensure their emails don't get sent to the Junk folder, as the Mail app generally exempts emails from people in your contacts list.

Also See Blocked Sender Options while in Mail Settings. Toggle the Switch to "On" and check off Move to Trash.

To start blocking spam in the iPhone you will have to press on the email and look for the subsequent options that appear, one of them is Block Contact. Press on it to block the contact.

That being said, if are receiving thousands of spam emails the best course is to create another email address and move what you have in the old email address to the new one and then delete the old email address.

iPhone User Guide