Good Evening Miss Snark,
I'm at the beginning of my writing career, have some short stories published, and working on a novel. As you can see, I'm miles away from the answer to this question. However, I also am one of those-gasp-writers who realize that no matter what style it is; this is a business. I'm wondering what you're thoughts are about a writer that can can come to you saying, "I've got a newsletter with 600 email addresses". I write literary fiction which, from my experience, doesn't always equal a balance of sales and art, but it's my argument that you do what you have to do to get your story-art-to as many people as possible and let them decide merit.
In the big picture, 600 email addresses isn't much, but does the fact that I'm a writer that considers the power of marketing make me a better candidate for a sale? What does readership mean to publishing houses?
Just because Miss Snark owns a thigh master does not mean she has svelte thighs. Having 600 email addresses does not mean you've got 600 people lined up slavering for the tome. Would that it did, Miss Snark would be capturing email addresses left and right.
However, your point is that you've started to think that your novel will not magically fly off shelves just cause you wrote it on a wing and a prayer. That's good. However, it doesn't matter one whit to Miss Snark when she squints at your pages. What makes a difference is if you've been part of a reading series that draws people; if you've published in magazines that have significant editorial oversight and competitive submissions; and, if you know BOOKSTORE owners and event coordinators.
A mailing list of 600 will most likely return at slightly higher than direct mail ratios: 2-3%. That means 12-18 people on your list will actually buy your book. That Grandma and your mom will buy multiple copies is a given.
Mailing lists are valuable for keeping track of your fans, and letting them know when the book is out, but it's one to one buying. I want authors who can reach bookstore owners/managers, cause those people can reach hundreds of buyers themselves. Do publishers look for people with built in marketing muscle? You bet. A fan club on a website is a beautiful thing, but it won't make the difference between yes and no for acquisitions.
Now, do I get a lot of novelists who cough up bookstore contacts on demand? More than you'd think; less than I want.
It's not a requirement that you come armed with lists. Just write well. That you know it's a business is good. That you understand the dynamics of marketing is better.