Think about how much time your customers spend in their email inboxes, and just how many personal details of their lives and relationships are contained there. Being admitted to what has become one of the modern world's most intimate spaces is a privilege for your business. How do you respect that trust and write emails that will delight rather than annoy your customers?

That's the topic of a hugely in-depth guide written by Jimmy Daly and recently published on the blog of email marketing startup Vero. The lengthy resource offers a whopping 20 tips to help you improve your emails and is full of examples from successful marketers to illustrate each point. Intrigued about what it contains? Here is a taste of 10 of the tips to get you started.

1. Create an exclusive club

Everyone likes to feel special. Use that to your advantage by highlighting that the recipient is getting a sneak preview or other perk not available to everyone. "Groove's Alex Turnbull takes a more direct approach. He emails new posts out to blog subscribers before they are available to people who read via RSS feeds or social media," notes Daly. Groove's emails explain that, "as a subscriber, you're getting this link about an hour before the post goes live on our blog's homepage."

2. Keep readers on their toes

Regularity is important in email marketing--if readers expect an email every Wednesday, that's exactly what you should give them--but that doesn't mean you should be satisfied if you don't surprise your customers at least a little bit. For example, "Quora's emails are like a box of chocolates--you never what you're gonna get next," says Daly, who also offers several other examples.

3. Tap into current events

"Urgency can seem desperate but timeliness is always powerful," says Daly. "Tapping into current events or seasonal trends is a good way to encourage readers to open and click emails since their value is passing."

4. Make people happy

If it works in real life, why wouldn't it work over email? "It's not that hard to make people happy. How many assets do you have that you could use to make an email recipient's day a little brighter? If you aren't ready to give something away, simply say something nice, tell an uplifting story, or just ask how they are," Daly reminds marketers.

5. Use the same subject line every time

This one is a little controversial given the importance many place on clickable subject lines, but Daly insists that "if your users trust you and your brand, it's a good idea to do this with transactional emails, like receipts, to make it very easy for users to find and reference them. But the tactic can be useful for newsletters and marketing emails too if you send them on a daily, weekly, or monthly basis." Plus, reusing the same subject line will certainly save you a lot of work coming up with catchy new ones each time.

6. Get personal

"The inbox is a intimate place, mostly used for personal conversation. Your presence there is a privilege, so do your best to write like a human. It's OK to talk about challenges, obstacles, and even failures. Readers can relate since they are likely facing similar situations with their own lives and businesses," writes Daly, who then offers several examples of emails that do this well.

7. Use buttons

Want people to click? Swap in-text links for buttons. Studies verify they're more effective, Daly points out. "Campaign Monitor, for example, got a 28 percent increase on click-thrus when they A/B tested emails with and without buttons," he reports.

8. Gamify a process

"Games, and their rewards, reinforce behavior that users want to repeat. Players earn achievements and unlock new levels, only to be sent out on new quests to earn and unlock more," says Daly. Marketers can use the same principles "to attempt to turn behavior into habit." Starbucks does this by awarding redeemable "stars" to loyal customers. TripAdvisor gives you a badge after a certain number of reviews.

9. Use referral codes

Confident in your brand? Then referral codes are a win for "the referrer, the referree, and the business," writes Daly. "According to ReferralCandy, 83 percent of satisfied customers are willing to refer a product or service but just 29 percent actually do. If you can motivate them to refer friends and family, it's makes conversion a breeze: 92 percent of consumers trust referral over ads."

10. Leverage your partner's brand power

"If your product or service integrates with other products or services, you are sitting on top of a gold mine of brand power," Daly reminds marketers. "Leveraging the power of other brands is a great way to accelerate the authority of your own," he says, offering this example: "Buffer has been creating case studies of some of their biggest customers and using the results as a tool to appeal to enterprise customers." One example is a helpful explanation of how Business Insider increased its Facebook presence using Buffer that reads like a combo testimonial/how-to guide.