WASHINGTON — Douglas J. Band, a longtime aide to Bill Clinton after he left the White House, sent an email to two of Secretary of State Hillary Clinton’s top aides with a “Very imp” message: The State Department needed to make one of its senior officials available for a conversation with a billionaire businessman — who also was a major donor to the Clinton Foundation.

The billionaire and donor, Gilbert Chagoury, wanted to speak to the State Department’s top official on Lebanon, Mr. Band wrote in the April 2009 email to the two aides, Huma Abedin and Cheryl Mills. Mr. Chagoury is a Nigerian-based hotel and real estate developer whose family is from Lebanon.

“As you know, he’s key guy there and to us and is loved in Lebanon,” Mr. Band wrote.

Ms. Abedin emailed back that the person Mr. Chagoury should speak with was Jeffrey Feltman, who had recently left his post as the United States ambassador to Lebanon. “I’m sure he knows him,’’ Ms. Abedin said in her email to Mr. Band. “I’ll talk to Jeff.’’

The exchange of emails emerged this week as a result of a lawsuit over emails sent on the Clinton family’s private server. They were immediately cited by conservative activists as more proof that Mrs. Clinton, while secretary of state, ignored an agreement to keep Clinton Foundation matters separate from her State Department duties.