Generally if something sounds too good to be true, it probably is. So unsolicited emails not coming via a good mailing list, offering to publish your book, inviting you to a conference somewhere nice with guaranteed acceptance, or guaranteed publication of papers in an online journal are generally warning signs that it's fishy. Either that or I've just ignored some really good opportunities in today's email... Look at other people's cvs in your field and see where they present and publish, and follow suit.