It's one of those military traditions that has stubbornly resisted the digital age: U.S. Marine recruits cannot make calls or send emails from boot camp, but they can write letters.

Some might bemoan the difficulty in sending their loved one an electronic holiday greeting this time of year, but a Crystal City start-up called Sandboxx has figured out a way around the problem.

The firm has developed an app allowing family members to type a note on their smartphone and have it turned into a printed letter and mailed the ­old-fashioned way.

The company's founders regard the app as more than just a mailing service. It's a bridge between the austere basic training environment and the ­smartphone-obsessed world recruits leave behind.

Among those leading the effort is Ray Smith, a decorated Marine general who earned the nickname "e-tool" for allegedly killing a man with a entrenching tool, essentially a shovel, when his service weapon malfunctioned at a critical moment. (He told Task and Purpose, a military blog, that the story was more of a "legend," which he defined as "a pack of lies that was built on an original kernel of truth.")

Smith, now 71, said he is devoting his retirement to getting his four-year-old start-up off the ground.

"I was supposed to be an old retired guy, but now I'm not," Smith said.

Smith has teamed up with Sam Meek, a Marine sergeant who left the military in 2007 to work for a Wall Street hedge fund.

The two met in 2011 and discussed a mutual interest for connecting "the extended military community," not just enlisted service members but also wives, husbands, parents, friends, siblings — anyone with a connection to the armed forces.

Meek said he first saw the power of social media when he returned from his first tour of duty in Iraq, made a Myspace page for his battalion and began to think over business opportunities.

"We thought to ourselves, 'What's the choke point for the audience?' It happens to be boot camp," Meek said.

So they founded Sandboxx in 2013 and launched the letters app the following year.

Ultimately, he said, their goal is to identify "friction points" in the military journey and look for creative ways to solve them. One of Smith's earlier ventures was a website for military spouses to stay in touch with deployed loved ones.

Smith sees a generational divide between the young, smartphone-toting men and women enlisting for the first time and the regimented military environment they become a part of at basic training. The culture shock of suddenly losing contact with the online world can take a toll on morale and interfere with training.

"It's a very traumatic shock to be forced off the grid that way," he said. "It's a really big deal for this younger generation."

The letters platform, available for Apple iOS and Android smartphones, has yet to attract a huge audience, but company executives say the users they do have are devoted, sending close to 900,000 letters since the app was released.

To give the app a boost for the holidays, the company is teaming up with Veterans of Foreign Wars, one of America's oldest veterans' associations. The VFW offered to pay for the next 5,000 letters sent through the app. So until the 5,000 cap is reached, anyone who sends a letter to a deployed service member through the app is notified that the letter is "courtesy of Veterans of Foreign Wars," and their fees are waived. A single message typically costs $3 but there are discounts available for buying in volume.

About 70 percent of the company's current letter volume comes from the Marine Corps, but the app is expanding fast. More and more people are using it to contact deployed Army soldiers and Air Force personnel, and the app will be offered to new Coast Guard members starting next month.

To build Sandboxx into a thriving technology company, the two former Marines teamed up to build the apps' features with a handful of young techies, many of whom work out of the Crystal City offices of 1776, a Washington-area start-up incubator.

The company grew with the help of $2 million from an undisclosed group of angel investors. More recently, the firm has raised another, larger tranche of funding that it hasn't disclosed. Advertising director Shane McCarthy said the company employs about 26 people full time and another 10 part-timers, with plans to hire 10 more in the near future. He said the company turned a profit last year.

While the letters platform has been the company's main product so far, Smith and Meek said they hope to eventually build a social media platform unique to the military community. They have already created another social media app called "units" based on the military's organizational structure. Any current or former member of the U.S. military can log in, put in their unit and year, and be connected solely with people from that unit and year.

The units app could be the basis for a larger platform to connect the military community.

"We have the force structure of all the military services built into our back-end," Smith said in a recent phone interview. "Units that have gone into mothballs, we have those units in our system."

For Smith, the work represents a labor of love, a continuation of his military service and a moneymaking venture.

"We believe we can do good — and make money, in plainspeak — while doing things that need to be done," Smith said.