[Tichy]

It seems a bit scary that forwarded mails can be changed. As a spammer, send something interesting and make it an ad when forwarded?

[zeckalpha]

Sending something interesting is a hard problem.

[Tichy]

Normal people forward silly jokes, or fake virus warnings, or fake cancer donation calls and stuff like that. Not sure it is that hard. Perhaps you could even monitor popular news sites for trending memes (for example trending YouTube videos).