Migrating 30,000+ Personal Emails to Proton Mail: A Field Report

Migrating decades of personal archived email from Thunderbird mbox format into Proton Mail via the Bridge IMAP interface. On Linux. From the command line. Because GUI add-ons kept crashing.

The Context

I’ve been online since the BBS days on a Commodore 64. Watched email evolve from FidoNet netmail through SMTP, POP3, IMAP, webmail, and now end-to-end encrypted providers. My personal archive contains messages from Outlook Web Access, Lotus Notes, Gmail, Yahoo, and half a dozen defunct providers. Each era wrote headers differently. Each client had its own interpretation of “RFC compliant.” The result is an archive that represents 20+ years of real-world email, which is to say, a complete and utter mess by any strict standard.