The emails Joomla sends are editable

Go to System, then under Templates pick Mail Templates. Every automatic email Joomla sends is listed there, with its subject and body, per language.

What is in the list

• New user registration, and the activation mail that follows it.
• Password reset, both the request and the confirmation.
• Contact form submissions, including the copy to the sender.
• Privacy requests, for the export and delete flows.
• Whatever your extensions have registered, if they use the core mail system.

Editing one

Open a template and you get a subject field and a body. Each template lists the placeholders it supports right there on the screen, which saves guessing. They look like {SITENAME} and {NAME}, and only the ones listed for that template will be filled in.

You can switch a template to HTML if you want formatting, but think about it first. Plain text arrives intact everywhere and never looks broken. Registration and password mails are exactly the sort of thing that should not look designed.

The part worth doing

Two changes take five minutes and are worth more than any redesign:

• Make the subject say what happened, in the site's own words. "Your account at Example is ready" beats the default every time.
• Add a line saying who sent it and what to do if it was not you. Password reset mails without that line get reported as phishing.

Check the sender first

None of this helps if the mail never arrives. The from address is in Global Configuration under Mail Settings, and it needs to be an address on your own domain with SPF and DKIM in order. A perfect template sent from a mismatched address lands in spam just the same.