EMLX
Apple MailApple Mail's per-message file format: one message per file, prefixed by its byte length and followed by an XML property list of Apple's own metadata.
Apple Mail stores each message as its own .emlx file. The structure is unusual but simple: the first line is a number — the byte length of the message that follows — then the message itself in standard RFC 5322 form, and finally an XML plist holding Apple's metadata, such as flags, mailbox identifiers and conversation information. Strip the first line and the plist and you are left with an ordinary EML file.
You will also meet .partial.emlx files. These belong to messages whose attachments were never downloaded from the server, so the file contains the headers and body but only placeholders where the attachments should be. If the account is gone, so are those attachments — a good reason to check before decommissioning an old mail account.
When Apple Mail exports a mailbox it does not produce .emlx at all: it writes a folder named Inbox.mbox containing a file literally called mbox, which is standard MBOX. Mbox Viewer resolves that package automatically and names the mailbox after the folder, so you see Inbox.mbox rather than a bare mbox.
Related terms
A single-message file in MIME format, containing headers, body, and attachments. EML files are widely supported across email clients and are useful for archiving or sharing individual messages.
A plain-text file format that stores multiple email messages concatenated together, each beginning with a "From " separator line. It is the format Google Takeout produces when you export your Gmail archive.
A file — such as a PDF, image, or spreadsheet — embedded in an email message and encoded as a MIME part, separate from the message body, intended for the recipient to save or open.