The leather-bound diaries of a 19th-century high-school principal in southwestern India are helping change how archivists around the world handle email.

As authors’ archives encompass fewer handwritten drafts and more digital documents, email remains a particularly sticky problem. That’s where the diaries, kept for 50 years by Sudheendra Hangal’s great-grandfather-in-law, come in. In 2004, when Mr. Hangal was working as an engineer for Sun Microsystems in India, he came across the journals in a trunk in his in-laws’ house...