Phishing detection

[ChristophWurst]

Is your feature request related to a problem? Please describe.

As a Nextcloud Mail user, I want the app to help me detect phishing emails. Some of these phishing emails are not easy to spot.

Describe the solution you'd like

Run some sanity checks on the email (to be brainstormed) and

1. Show a warning
2. Disable external links
3. Offer to move the message to spam

Work packages

• [x] Research, brainstorming and goal definition
  • [x] Compare sender with sender label
  • [x] Trusted domains
  • [x] DKIM
  • [x] BIMI https://github.com/nextcloud/mail/pull/5784
• [x] Implementation
  • [x] anchor tag text vs link checking #9994
  • [x] Highlighting messages with a different reply-to #9610
  • [x] check full name address against contacts #9610
• [x] #9891
• [ ] Unit test for link checking
• [ ] Admin docs if applicable
• [ ] User docs

Follow-up

• [ ] Add a setting to disable HTML emails by default
• [ ] Add quick actions (example: add email address to contact)
• [ ] Check emails for selectively applied CSS on forwards context : https://www.schneier.com/blog/archives/2024/04/security-vulnerability-of-html-emails.html

Out of scope

• resolving domain names
• Build a process to integrate against configurable Lists
• SPF
• Preconfigured trust custom header tags

Describe alternatives you've considered

N/a

Additional context

https://github.com/nextcloud/mail/issues/7864

[kaffeeringe]

Really important topic. I personally would like to get rid off all the fake DHL-notifications. But this is an essential feature for organisations.

[kaffeeringe]

Maybe add a hint if a mail comes from an address that is neither in "contacts" nor has send any mails before.