The easiest way for paid ProtonMail users is the ProtonMail Import-Export App, which helps you import your MBOX file in a few clicks.
If you want more control to import MBOX to ProtonMail, Advik MBOX Converter is a great choice. You can convert MBOX files to EML format or directly import them to ProtonMail via IMAP. The software has dual options to add files—Add Files for picking specific MBOX files and Add Folder for loading all files in bulk. It’s 100% safe and keeps all your emails, attachments, and folder structure exactly as they were. Plus, the tool is simple to use, works on any Windows computer, and doesn’t require any special skills. You just select your files, choose ProtonMail as the destination, enter your login details, and click Convert—done!
