How do I keep emails permanently

The only mostly permanent way is to have a local copy. Make a screen cap, or cut and paste the contents into a word document or print it out.

In theory the mail will remain on the server forever, however stuff happens. A flood, fire or simple hardware failure could wipe your mail out. So the 'safest' storage way is something you control.

The exact how depends on your mail provider and what they can allow you to do. For example my work e-mail is microsoft exchange and I can download mails as pdf files. But that's not always an option with every provider.

Whatever you do you'll need to do it outside the mail app. Perhaps your mail provider has a webmail interface that you can use.