I can't delete many emails. How can I fix this?

Before moving forwards with the issue - suggest updating Monterey with the latest version 12.1 released Dec 13, 2021 - It brings some bug fixes and may be related to your issue .

Suggest doing a Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS. before attempting the update.

The contents of the message is gone, but the original emails stays on my email page.

Try rebuilding the Mailbox. Select the Mailbox (Inbox) of the account, then choose Rebuild from the Mailbox menu.

Who is the email provider? Gmail makes it almost impossible to completely delete an email.

Writing an effective Apple Support Communities question

Then do come back with some Details and Context

We may better understand the overall situation

speed1313 wrote:

Third Party Mail Server.. Advertisements, charity organizations, etc.

What does Connection Doctor Report ?

Same place as suggest previously in suppled picture

Are the e-mail coming from an iCloud Account as in iCloud as Mail Server or from a Third Party Mail Server ?