Working in such an exciting, fast-paced channel, we email marketers have the privilege of constantly discovering, testing and revising industry best practices. Through our accumulated efforts, we've gained a solid grasp on best bets in areas ranging from sending frequency to image use. As we work with what we've learned, it's important for us to keep in mind that "best practices" still leave some room for creativity and on-brand innovation. It's fun to look at email from brands that defy the best practices and to consider how it's important to view the "rules" holistically, evaluating how they apply to our subscribers in particular.

Here are some examples of when following best practices works -- and when the "rules" can be bent for the better:

Preview Pane Optimization

• Costco: As a value brand, it's key for Costco to deliver information efficiently. Costco packs a punch in its preview pane by including as much detailed information as possible.

• Barneys New York: In contrast, brands like Barneys need to optimize the email channel to deliver the same luxury that its products promise. The huge images in this email are meant to be viewed in whole to create a striking impression; they're selling experience. In this case, we're less interested in seeing delivery cost details and more interested in seeing diamonds on the soles of those Margielas.

Subject Line Length

• Nordstrom: While there aren't "subject line absolutes," most marketers aim for 35-45 characters that clearly indicate message content and highlight special offers. Nordstrom's subject line, "Get FREE Shipping with Any Shoe Purchase" entices opens with the promise of an offer and sets up the subscriber for the sight of great shoes.

• Anthropologie and Free People: Both of these brands embrace an artistic, whimsical voice, and it shows in their subject lines, here "Dust off the dog ears" and "so exclusive" respectively. With lower character counts than most subject lines, they stand out in the inbox. While these subject lines aren't direct or clear about the content of the email, they do reinforce each brand's image -- a tactic subscribers definitely respond to.

Sale Email Creative

• J. Crew: Simple, copy-based messages like J. Crew's are representative of most sale mails. Maintaining straightforward creative in sale messaging is an effective way to deliver straightforward savings.

• Pottery Barn Kids: For some aspirational brands like Pottery Barn Kids, the best way to reinforce branding is to carry lifestyle photography across sale messages. This approach balances the emphasis on sale pricing with the high-end lifestyle offerings that subscribers expect.

Welcome Messages

• Gap: This welcome message hits the mark. It's clearly branded, includes a special offer and an invitation to edit preferences, gets the subscriber shopping and sets the tone for the rest of the Gap email program.

• Pandora: By our best practice standards, Pandora's copy is long, and the message isn't direct (it doesn't steer subscribers back to the Web site). Because Pandora is a free service that doesn't need to upsell to already-engaged subscribers, the company uses its welcome message to highlight its human face and to introduce its accessible customer service.

Calls to Action

• REI: As in this REI CTA "Shop REI Travel Collection," calls to action are usually best as big, bold buttons placed above the fold, with clear wording that lets subscribers know exactly what they should do and what will happen when they click.

• Backcountry: Backcountry CTAs often employ playful copy that represents the company's irreverent voice, like this "Light It Up" CTA. Similarly, Anthropologie often selects indirect and fanciful CTAs to match its voice, like "Fa-la-la Filter."

The best practices we've all found and honed are valuable guides toward email success, but these strong brands exemplify why we should beware slavishness to our "rules." As email marketers, we should always look for ways to customize experiences to the specific needs and tastes of our unique subscriber bases.