We do not encourage sending bulk email via your website. There are extensions that provide this kind of functionality and it has been suggested that we create a plugin or bridge to work with one of them. We prefer to leave bulk mailing to the experts and would suggest Constant Contact, iContact, MailChimp, or any number of other mailing list vendors that are available. Since you can export the contacts from Grumpy Contacts, you should be able to import that list into the mailing system of your choice.