Hacker News Reader

I argue that mail clients should have a blacklist option: When logged in as account X, NEVER permit me to send email to email address Y.

The blacklist can be compromised, which is a larger vulnerability than having no blacklist.

I blacklist is modified, notify me via SMS (the same way Google/Facebook can handle you not able to get into your account).

It's not about modifying; just reading the addresses you don't want to send mails to is informing enough.

Don't display them to the user without two-factor authentication then. Everything can be compromised at some point.

Save only the hash, or use a data structure like a bloom filter.