Email marketing is crucial to hitting your holiday numbers.

Online sales reached $107.1 billion for Q4 2015, with email marketing driving 20% of that revenue. However, the competition for the inbox is fierce; the email volume of Q4 2015 increased by 23% from what it was in 2014, and 2016 is projected to grow by similar amounts.

Last week, I caught up with my friend Carl Sednaoui to talk about which email strategies will work best around the holidays. In the past, Carl and I have collaborated on email marketing projects for Y-Combinator startups, and he's also given me great advice about survey emails.

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While my expertise is with writing cold emails for Business-to-Business companies, Carl is an e-commerce email marketing expert. He's the Director of Marketing for MailCharts--an email database which helps online retailers like Carter's and Lucy Activewear plan and optimize their email segmentation efforts.

Carl recently released a fascinating 100-page report on email marketing strategies for the holidays in collaboration with Litmus, AWeber, Return Path, Movable Ink, and other key players in the space. After analyzing holiday email programs from more than 5,000 companies, including the 1,000 largest U.S. web merchants, Carl and his colleagues uncovered some really interesting insights.

Here are some of Carl's favorite takeaways for your holiday email marketing strategy:

#1: Use percentage discounts for promotions

Through analyzing the email program of the largest 1,000 U.S. web merchants, MailCharts found that offering subscribers a "percent off" is the most popular promotional strategy for the holidays.

To illustrate this point, here's a breakdown of how top retailers promoted their products in Q4 2015:

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The reason why "percent off" is so popular is better explained with "the Rule of 100." This rule states that when something costs under $100, percentage discounts seem larger than absolute ones. For goods over $100, absolute discounts seem larger than percentage ones.

#2: Put promotions in the subject line

The same data from the top 1,000 U.S. web merchants showed that 59% of Q4 email promotions are mentioned in the subject line. Not only that, most promotions are mentioned within the first 14 characters!

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If you want to make sure subscribers are aware of your promotion, mention it early in the email subject. Even if some subscribers won't open your email, the offer might still register with them. Take Ann Taylor's "50% Off Everything Means EVERYTHING." and Forever 21's "Final Hours to Get Up to $100 Off" emails, for example. Both used a clear and click-worthy subject line.

#3: Send a Cyber Monday sneak peek

MailCharts' analysis of more than 5,000 companies' emails uncovered that most companies do not give subscribers a "Cyber Monday sneak peek." Therefore, merchants who do send these "sneak peek" emails a few days before Black Friday have much better odds of having their email be seen before consumers' inboxes get crowded.

To give some context, less than 17% of Cyber Monday emails are sent the week before the holiday. Instead, 56% of emails go out on the holiday itself. Compare this to Black Friday, where only 30% of Black Friday emails are sent on the actual holiday.

If you send a Cyber Monday sneak peek email a week ahead, not only will you build anticipation with your audience, but you'll beat your competitors to your customers' inboxes.

#4: Gift cards are very popular on Christmas

Gift cards have already been the most requested gift of the season for 9 years in a row.

Since your subscribers are likely to receive store-agnostic gift cards for Christmas (e.g. Visa or Amex gift cards) from their friends and family, you should take advantage of that.

After their Christmas gifts are unwrapped, send them an email that encourages them to buy something from you with those gift cards. You can frame it as an opportunity for them to "treat themselves," and get that item that they badly wished for, but didn't find under the Christmas tree.

#5: Optimize your emails for mobile

Even though 56% of emails are opened on a mobile device, MailCharts discovered that only 40% of emails sent by the top 1,000 U.S. web merchant are mobile optimized.

If you're still sending non-mobile optimized emails, you're stuck in the dino era, and are ruining your customer's experience with your brand on their mobile inboxes.

#6: Keep your subject lines to under 45 characters

The top 1,000 web merchants send subject lines with an average of 43 characters. This makes sense, considering that most email clients display around 45 characters in the subject. Here are the different character lengths displayed for the most common browsers:

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Bottom line: keep your subject lines short; aim to stay under 45 characters.

#7: Speed up the timing on your cart abandonment emails

The holiday season means shorter purchase consideration times. This can be caused by limited-time offers, concerns about out-of-stock issues, or the pressure of ordering before hard deadlines.

Because of all this, you need to send your cart abandonment emails sooner. This will help you maximize cart abandonment email conversions, especially on marque shopping days like Black Friday and Cyber Monday, when many purchase decisions are made quickly.

#8: Use back-in-stock emails to prevent lost sales

In the last quarter of the year, when the discounts are large, and consumer spending is even larger, it's only natural to sell out of popular items. When this happens, consider sending a back-to-stock notification email to subscribers who previously browsed or attempted to purchase those popular out-of-stock products once you've replenished your inventory.

Combining great customer service with some promotion will help you increase sales, while remaining extremely relevant to your subscribers. It's a win-win for everyone.

#9: Don't sell your way to the spam folder

Consumers received 50% more promotional emails during the holidays than they saw earlier in the year. While driving sales is crucial to your business, that's impossible to do if your emails don't even reach the inbox.

With subscriber engagement playing an increasing role in mailbox providers' filtering decisions, you need to test different aspects of your email program to see what increases subscriber engagement, which has a positive impact on your deliverability. Engagement metrics to monitor include opens, clicks, and purchases.

It's normal to increase email volume in Q4 but don't ramp frequency without paying attention to subscribers' engagement.

#10: Send emails during unpopular holidays

Free Shipping Day contributes over $1 billion in online sales, yet very few companies take advantage of this holiday. Make sure to include this holiday in your Q4 plan.

The same can be said about Winter Solstice, which takes place on December 22. If you can ship items fast enough, this is a fantastic opportunity to promote your best selling items in time for Christmas.

Follow these tips, and your e-commerce email marketing program will shine brighter than Rudolph's nose. And if you want more tips to improve your holiday campaigns, check out Carl's 2016 Holiday Email Marketing Report.