14 Jul How frequently people click on “This is Spam” on web-based e-mail services.
For this reason you shouldn't click "This can be Spam" since it affects the sender's reputation overall unless it really is spam - clicking that will cause other people to stop getting it as well.
Sender can indicate:
The e-mail address when you look at the "From: " industry of a message. (Unreliable, because it could be spoofed, and sometimes is by spammers. )
The internet protocol address of this computer delivering the email. (Unreliable, since botnets regularly deliver from thousands and thousands of compromised PCs owned by typical customers. )