New York startup Sailthru has built a business helping clients such as Thrillist, Huffington Post and American Media customize e-mail, online and in-app content to increase stickiness and user loyalty. More recently, the four-year-old company has focused on expanding into the e-commerce sector, with clients including Soap.com, Citysearch/Urbanspoon, Totsy.com and OpenSky adopting its personalization technology.

Last month, Sailthru also acquired Frame, which makes software that allows e-commerce businesses to optimize their Web presence for the iPad. Continuing in that vein, Sailthru has now acquired Seamless Receipts, which started by giving retailers a way to send electronic receipts to in-store customers and now provides a set of tools including targeted promotions and consumer shopping analytics.

Through the deal, Sailthru is picking up the five-person team behind Seamless Receipts in New York as well as clients including Burton Snowboard, Oakley and Tumi. The startup will continue to operate as a separate unit under the Sailthru umbrella. Terms of the transaction were not disclosed. 

“This acquisition helps us gain access to more top-tier retail clients while adding invaluable engineering talent and industry insight that will enhance our growing e-commerce initiatives,” said Sailthru CEO Neil Capel in an interview.

He added that Seamless Receipts’ technology will advance its goal of sending highly targeted e-mails that cut through the clutter of crowded in-boxes. “We believe that sending more targeted messages to subscribers, less frequently, will result in higher conversions and keep consumers engaged,” he said.

Capel pointed out that brick-and-mortar retailers are one of the last industries to adapt to the digital era, creating a big opportunity for the company to exploit. Companies with e-commerce business now account for more than a third (35%) of Sailthru’s client base and e-commerce is driving 15% monthly revenue growth for the company.

Sailthru typically forms one-year, software-as-a-service (SaaS) contracts with clients, according to their volume of e-mail and Web site traffic. What results are they seeing? Across its client base, the company says it has delivered a 12.3% increase in revenue per unique users, a more than 200% uptick in time spent on sites, and email open rates topping 35%.

“We’re able to really create customer loyalty from being able to deliver very customized content to the in-box and to sites,” said Capel. In a testimonial, Thrillist said it uses Sailthru’s APIs (application programming interface) to track a user’s interaction in email and on the Web to tailor the millions of emails it sends to subscribers every day.

Capel noted that the company’s system doesn’t rely on third-party data providers and that it doesn’t sell any client data to outside companies.

Sailthru raised $8 million in first-round funding last September led by RRE Ventures, which has helped finance its recent acquisitions. Business Insider reported earlier last week that sailthru is now in the process of raising a second round of $20 million to $25 million. (Business Insider is a client of Sailthru and Sailthru CTO Ian White is a former BI employee.)

Capel said the company has been in discussions with several venture capitalists, but that there is nothing definitive to say as far as a new round of funding. He added that Sailthru’s current revenue is sufficient to support its growth. Among other initiatives, the company is extending its service for creating iPad-friendly versions of e-commerce sites to mobile phones as well in the next two months.