Hi Takayuki. This morning I updated two plugins:
Contact Form 7 from v3.3.3 to v3.4
WordPress SEO v1.4.4 to v1.4.6
This problem started with v3.4. I've disabled plugins including SEO and it doesn't make a difference. The "From" is still incorrectly set to the WordPress "General Settings" email address instead of the sender's email.
Thanks.