What is the best way to do a mass mailing to my customers?

I believe that the following QuickBooks Knowledge Base article can assist you with your question.
Title: Sending e-mail to multiple contacts
Link: http://support.quickbooks.intuit.com/support/Pages/KnowledgeBaseArticle/1...

I hope this helps.
 

By email!

I found that if I emailed all my clients from Outlook at once, we would likely be flagged as spam by automatic filters that identify emails with multiple (lots) emails in the "To" field. So, my IT guy suggested using ConstantContact.com. The more I research, the more I have found this to be a reputable, solid resource for mass emailings. They offer a free subscription for 60 days for up to 100 recipients on your list, $15 after that, or $15 (super cheap!) for over 100 contacts that, up to 500 contacts. Additional people on your list, more money, but still, SO CHEAP.

Their interface is easy to use; you can have their logo removed from the bottom of emails, but they require use of a standard "Do you wish to unsubscribe" message at the bottom of your emails, which really, is just plain responsible, and in keeping with their fervent "NO SPAM" policies. I found the easiest template to use was the Newsletters - Basic template - I didn't need frills, just a basic email that I could upload some graphics (jpeg) into.

Here's the link: http://www.constantcontact.com.... I've called their customer service several times and every time have spoken with a real person, very helpful, and virtually no on-hold time. Can't say enough about them!