Under the deal announced after the close of markets, Open Text will pay $7.25 per share in cash for the provider of electronic messaging and business services.
Shares in Atlanta-based EasyLink, which has about 550 employees, were up 46 cents at $6.36 on the Nasdaq market on Tuesday, still well below the offer price.
"We see strong opportunities for our mutual customers, partners, employees and product road map," Open Text chief executive Mark Barrenechea said in a statement that came as the company also reported slightly weaker quarterly profits.
"EasyLink has demonstrated operational discipline and earnings as reflected in their financial results."
The deal, which is expected to close by mid-to-late summer, is subject to EasyLink shareholder approval as well as other regulatory and closing conditions.
EasyLink's board unanimously approved the proposed deal.
"After a thorough review of a broad range of alternatives to enhance stockholder value, our board of directors concluded that the best available option was a merger with OpenText," EasyLink chairman Kim Cooke said in a statement.
"We are pleased that this transaction appropriately recognizes the value of EasyLink's relationships, technology and solutions, while providing our stockholders with an attractive cash premium for their investment."
The acquisition came as Open Text reported a quarterly profit of $34.8 million or 59 cents per diluted share, down from $35.8 million or 61 cents per diluted share a year ago.
Revenue in what was the company's third quarter totalled $292.3 million, up from $263 million.
Open Text sells software used by companies to manage their electronic documents.