To find what campaigns bring in the best ROI, O’Keefe suggests that marketers test as many aspects of their campaign as possible, including frequency, number of pages and types of mail they’re sending. He also suggests that marketers get a baseline of their efforts, then test their campaign by holding off on sending mail to certain segments of customers. This can help marketers understand the true value of that segment. O’Keefe says that many marketers get nervous about losing touch with a potentially important customer group, but he believes holding off is one of the best ways to get statistically significant data on the ROI of direct mail.
A web-based interface is often available to allow people to subscribe, unsubscribe, and change their preferences. However, mailing list servers existed long before the World Wide Web,[1] so most also accept commands over email to a special email address. This allows subscribers (or those who want to be subscribers) to perform such tasks as subscribing and unsubscribing, temporarily halting the sending of messages to them, or changing available preferences - all via email. The common format for sending these commands is to send an email that contains simply the command followed by the name of the electronic mailing list the command pertains to. Examples: subscribe anylist or subscribe anylist John Doe.
Purchased lists are ineffective, and they impact everyone else who uses Mailchimp, too. If you send emails to a list of people whose contact info you bought, many of the emails will get identified as spam. Some spam filters will flag a campaign if anyone with the same IP has sent spam in the past. When you use Mailchimp, your email is delivered through our servers, so if one person sends spam, it could prevent other users’ emails from reaching inboxes. But by forbidding Mailchimp users from using purchased lists, we increase deliverability for everyone.
Particular groups of customers can be targeted or even individuals. Offering individual customers special deals on merchandise and/or services on the customer's birthday, for instance, is one example of email marketing personalization. (A restaurant might send an email to customers on their birthday offering 50% off an entree,) Email marketing helps a business develop and maintain a relationship with a customer over time that hopefully results in increased sales and increased customer loyalty. 
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