Building MIME e-mail messages in Rust

2 min readJan 25, 2022

Back in November we released mail-parser, a Rust library to parse MIME e-mail messages of any complexity. Today we are proud to announce the release of mail-builder, a simple yet powerful library to build RFC5322 e-mail messages with MIME support.

Using mail-builder is straightforward:

    // Build a simple text message with a single attachment
    let mut message = MessageBuilder::new();
    message.from(("John Doe", "john@doe.com"));
    message.to("jane@doe.com");
    message.subject("Hello, world!");
    message.text_body("Message contents go here.");
    message.binary_attachment("image/png", "image.png", &[1, 2, 3, 4]);    // Write message to memory
    let mut output = Vec::new();
    message.write_to(&mut output).unwrap();

More complex messages with grouped addresses, inline parts and multipart/alternative sections can also be easily built:

    // Build a multipart message with text and HTML bodies,
    // inline parts and attachments.
    let mut message = MessageBuilder::new();
    message.from(("John Doe", "john@doe.com"));    // To recipients
    message.to(vec![
        ("Antoine de Saint-Exupéry", "antoine@exupery.com"),
        ("안녕하세요 세계", "test@test.com"),
        ("Xin chào", "addr@addr.com"),
    ]);    // BCC recipients using grouped addresses
    message.bcc(vec![
        (
            "My Group",
            vec![
                ("ASCII name", "addr1@addr7.com"),
                ("ハロー・ワールド", "addr2@addr6.com"),
                ("áéíóú", "addr3@addr5.com"),
                ("Γειά σου Κόσμε", "addr4@addr4.com"),
            ],
        ),
        (
            "Another Group",
            vec![
                ("שלום עולם", "addr5@addr3.com"),
                ("ñandú come ñoquis", "addr6@addr2.com"),
                ("Recipient", "addr7@addr1.com"),
            ],
        ),
    ]);    // Set RFC and custom headers
    message.subject("Testing multipart messages");
    message.in_reply_to(vec!["message-id-1", "message-id-2"]);
    message.header("List-Archive", URL::new("http://example.com/archive"));    // Set HTML and plain text bodies
    message.text_body("This is the text body!\n");
    message.html_body("<p>HTML body with <img src=\"cid:my-image\"/>!</p>");    // Include an embedded image as an inline part
    message.binary_inline("image/png", "cid:my-image", &[0, 1, 2, 3, 4, 5]);    // Add a text and a binary attachment
    message.text_attachment("text/plain", "my fíle.txt", "Attachment contents go here.");
    message.binary_attachment(
        "text/plain",
        "ハロー・ワールド",
        b"Binary contents go here.",
    );    // Write the message to a file
    message
        .write_to(File::create("message.eml").unwrap())
        .unwrap();

Nested MIME body structures can be created using the body method:

// Build a nested multipart message
    let mut message = MessageBuilder::new();    message.from(Address::new_address("John Doe".into(), "john@doe.com"));
    message.to(Address::new_address("Jane Doe".into(), "jane@doe.com"));
    message.subject("Nested multipart message");    // Define the nested MIME body structure
    message.body(MimePart::new_multipart(
        "multipart/mixed",
        vec![
            MimePart::new_text("Part A contents go here...").inline(),
            MimePart::new_multipart(
                "multipart/mixed",
                vec![
                    MimePart::new_multipart(
                        "multipart/alternative",
                        vec![
                            MimePart::new_multipart(
                                "multipart/mixed",
                                vec![
                                    MimePart::new_text("Part B contents go here...").inline(),
                                    MimePart::new_binary(
                                        "image/jpeg",
                                        "Part C contents go here...".as_bytes(),
                                    )
                                    .inline(),
                                    MimePart::new_text("Part D contents go here...").inline(),
                                ],
                            ),
                            MimePart::new_multipart(
                                "multipart/related",
                                vec![
                                    MimePart::new_html("Part E contents go here...").inline(),
                                    MimePart::new_binary(
                                        "image/jpeg",
                                        "Part F contents go here...".as_bytes(),
                                    ),
                                ],
                            ),
                        ],
                    ),
                    MimePart::new_binary("image/jpeg", "Part G contents go here...".as_bytes())
                        .attachment("image_G.jpg"),
                    MimePart::new_binary(
                        "application/x-excel",
                        "Part H contents go here...".as_bytes(),
                    ),
                    MimePart::new_binary(
                        "x-message/rfc822",
                        "Part J contents go here...".as_bytes(),
                    ),
                ],
            ),
            MimePart::new_text("Part K contents go here...").inline(),
        ],
    ));    // Write the message to a file
    message
        .write_to(File::create("nested-message.eml").unwrap())
        .unwrap();

The library is available at https://crates.io/crates/mail-builder and the documentation at https://docs.rs/mail-builder.

Building MIME e-mail messages in Rust

Written by Stalwart Labs