The Internet mail architecture and the MIME multipart subtypes

One message, five components

E-mail looks like a single hop — you press send, it arrives — and that impression is exactly what the exam punishes. A message crosses five named components, each of which is somebody's representative, and the discrimination the paper tests is whose.

Worked example. Priya, at kestrel.example, writes to Omar at heron.example.

1. Priya composes in her mail client. That client is her Message User Agent (MUA) — the user's representative inside the mail service, typically a client e-mail program running on her own computer, though on a campus it may equally be a local network e-mail server. Hers is the author MUA: it formats the message and submits it. Omar's client is the recipient MUA: it stores or displays what arrives. Both ends of the journey are MUAs.
2. Her client hands the message to kestrel.example's Mail Submission Agent (MSA), over SMTP. The MSA is the hosting domain's representative: it applies the domain's own policies on top of whatever the Internet standards require — rate limits, authentication of the submitting user, rewriting of a malformed header.
3. The MSA passes it to a Message Transfer Agent (MTA). An MTA relays a message for exactly one application-level hop, the way a packet switch or an IP router moves a packet one hop closer. It makes a routing assessment, hands the message to the next MTA, and — this is the detail that identifies it — adds trace information to the message header as it goes. A message crossing the public Internet usually passes through a sequence of MTAs.
4. The last MTA hands the message to the Mail Delivery Agent (MDA) at heron.example, which transfers it out of the handling service and into storage.
5. It lands in the Message Store (MS) — the long-term store, which may live on a remote server or on Omar's own machine. Omar's MUA fetches it from there using POP or IMAP, not SMTP. SMTP pushed the message across; POP and IMAP pull it out of the store.

The collection of MTAs is the Message Handling Service (MHS), and the point of naming it is that the MHS creates a virtual MUA-to-MUA exchange environment: the two user agents behave as though they were talking directly, and everything in between is transport. Each provider along the way is an Administrative Management Domain (ADMD) — a department relay, an enterprise relay, an ISP's public mail service — and each ADMD sets its own policies and makes its own trust decisions, which is precisely why a message crossing an organisational boundary is treated differently from one that never leaves the building.

MIME, and the four multipart subtypes

The original mail format assumed plain ASCII text. MIME extends it with header fields that describe the body's format and encoding, and with content types that standardise multimedia representations — text, image, audio, video. The type worth memorising is multipart, whose four standard subtypes are separated by two questions: are the parts the same information or different information, and does their order matter.

• multipart/mixed — several independent parts, to be delivered and presented together, in the order given. A memo with two attachments. This is the everyday one.
• multipart/parallel — the same idea with the ordering requirement dropped. A viewer may render the parts simultaneously, or in whatever sequence it likes: an audio track alongside the slide it belongs to.
• multipart/alternative — the parts are different renderings of the same information. A plain-text version and an HTML version of one message; the recipient's software picks the best form it can display and shows only that.
• multipart/digest — like mixed, but each part defaults to message/rfc822: it is a collection of forwarded messages, such as a mailing-list digest.

Read that list again for the one word that carries a whole exam question. Mixed and parallel differ in nothing except order significance — and parallel is the one where order does not matter. A learner who reaches for mixed because it is the familiar one gets it exactly backwards.