Jerry Yang’s memos to fellow Yahoos are becoming an almost daily ritual at the Sunnyvale, Calif.-based company. The chief executive officer of the Internet pioneer — which just turned down Microsoft’s (MSFT) $44.6 billion buyout bid — sent another letter to his employees Monday, explaining the board’s decision.

“As you’ll see from the news release we issued today, our board of directors has reviewed Microsoft’s unsolicited proposal with Yahoo’s management, financial and legal advisers,” Yang wrote in the e-mail, filed with the Securities and Exchange Commission. “After a careful evaluation, the board has unanimously concluded that the proposal is not in the best interests of Yahoo and our stockholders.”

Yang added that Microsoft’s bid “substantially undervalues” the company and that the board would continue to evaluate all of its “strategic options.” It’s been reported that those options include a partnership with Google (GOOG), though it’s not clear how attractive that would be to Yahoo (YHOO), or whether such a move could even clear potential antitrust hurdles.

Yang continued his e-mail with a pep talk to employees, saying they “deserve credit for the tremendously valuable business we have built.”

“All of us in management, as well as the members of the board, deeply appreciate and respect what you have done and continue to do in order to maintain and enhance Yahoo’s leadership position in the online world,” wrote Yang. The CEO then went on to list Yahoo’s key assets — including its global brand, heavily-trafficked content sites and investments in computing infrastructure like its Panama search engine.

The danger of a protracted struggle over Yahoo is that key Yahoo employees will be cherry-picked by other tech companies. Mid-level Yahoo execs are already getting multiple offers from Oracle , Google and Salesforce.com , according to Global Equities Research analyst Trip Chowdhry.

No surprise then that Yang signed off by saying, “Of course, our most important resource is you: the thousands of creative, passionate and committed Yahoos who are executing our strategies to deliver value for users, advertisers, publishers — and stockholders.”