You may have noticed some 419 Nigerian scammers are using incredibly lame emails.

These messages are sometimes full of spelling and grammatical errors, and generally scream "This is a scam" to most computer users who are the least bit suspicious.

Turns out that's by design: http://wsj.com/article/SB10000872396390443931404577548813973...

Excerpt:
It costs the scammers virtually nothing to spam the world, but it costs them a lot (especially in terms of time) to conduct all the follow-ups necessary to reel a sucker all the way in. The people behind "Captain Mbote" spent six months pursuing their quarry before he started wiring money to them.

A proposal offering a more realistic scenario might generate more replies, but most of them wouldn't pan out. The effort of sorting through them to find the real suckers would undermine the scheme's profitability. Instead, by screaming "This is another absurd instance of the familiar Nigerian scam," the fraudsters are filtering out what to them is spam—responses from suspicious people they don't want to deal with—and "letting through" only those most likely to play along. The fewer potential victims in the world, the more precisely the scammers must target them, and thus the more absurd and easy-to-spot the attacks should be.

Pretty clever, eh?