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mail_struct : Minimalist Email Structure for Rust

mail_struct is a lightweight Rust library designed to define a clear and efficient structure for email messages. It provides optional integration with bitcode for efficient encoding/decoding and mail-send for SMTP transmission with domain-based grouping, making it a versatile choice for email handling in Rust applications.

Table of Contents

• Features
• Usage
• Design Philosophy
• Tech Stack
• Directory Structure
• API Documentation
• Historical Context

Features

• Core Structure: Defines Mail, UserMail, and HostUserLi structs to represent email data with type-safe domain grouping.
• Serialization: Optional encode and decode features using bitcode for high-performance binary serialization.
• SMTP Integration: Optional send feature with domain-based recipient grouping for efficient email delivery.
• Type Safety: Leverages Rust's type system with custom types like HostUserLi to ensure data integrity and prevent misuse.

Usage

Add mail_struct to your Cargo.toml:

[dependencies]
mail_struct = { version = "0.1.18", features = ["send", "encode", "decode"] }

Creating and Grouping Emails by Domain

use mail_struct::Mail;

#[cfg(feature = "send")]
async fn example() {
    // Mail::new() automatically:
    // - Normalizes and validates email addresses using xmail::norm_user_host
    // - Filters out invalid email addresses
    // - Deduplicates recipients
    // Returns Option<Mail>, None if no valid recipients
    let mail = Mail::new(
        "sender@example.com",
        vec![
            "user1@gmail.com",
            "user2@yahoo.com",
            "user3@gmail.com",
            "user1@gmail.com",  // Duplicate - will be removed
            "invalid-email",     // Invalid - will be filtered out
        ],
        b"Hello, this is a test email!",
    ).unwrap(); // Handle None in production

    // Directly iterate over &mail to get MailMessage grouped by domain
    for msg in &mail {
        println!("Sending to domain: {}", msg.domain);
        
        // 1. Try batch sending to all recipients in this domain
        // MailMessage implements IntoMessage trait, can be passed directly to client.send()
        if let Err(e) = client.send(&msg).await {
            println!("✗ Batch send failed: {}, sending individually", e);
            
            // 2. If batch send fails, use IntoIterator to send individually
            // This uses a custom zero-allocation iterator
            for individual_message in msg {
                match client.send(individual_message).await {
                    Ok(_) => println!("  ✓ Individual send successful"),
                    Err(e) => println!("  ✗ Individual send failed: {}", e),
                }
            }
        } else {
            println!("✓ Batch send successful");
        }
    }
}

Design Philosophy

The library follows a separation of concerns principle. The core lib.rs defines the data structures (Mail, UserMail), keeping the base dependency footprint minimal. Functionalities like serialization and sending are gated behind feature flags (encode, decode, send), allowing users to opt-in only for what they need.

When the send feature is active, the send.rs module implements IntoIterator for &Mail, returning MailMessage instances grouped by domain. This optimization reduces the number of SMTP connections needed and improves delivery efficiency. Additionally, MailMessage also implements IntoIterator, providing a zero-overhead fallback mechanism that ensures individual delivery to valid recipients when batch sending fails due to invalid recipient addresses.

Tech Stack

• Rust: Core language.
• xmail: Email validation and normalization.
• bitcode (Optional): For fast binary encoding and decoding.
• mail-send (Optional): For SMTP message construction and sending.

Directory Structure

.
├── Cargo.toml          # Project configuration
├── README.md           # Main documentation
├── readme              # Documentation in specific languages
│   ├── en.md           # English README
│   └── zh.md           # Chinese README
├── src
│   ├── lib.rs          # Core struct definitions and feature gates
│   ├── host_user_li.rs # HostUserLi type for domain-based recipient grouping
│   └── send.rs         # SMTP message logic (feature: send)
└── tests
    └── main.rs         # Integration tests

API Documentation

struct Mail

Represents a basic email message.

• sender_user: String: The user part of the sender's email address.
• sender_host: String: The domain part of the sender's email address.
• host_user_li: HostUserLi: Recipients grouped by domain using a type-safe wrapper around HashMap<String, HashSet<String>>.
• body: Vec<u8>: The raw body content of the email.

struct HostUserLi

A type-safe wrapper for domain-based recipient grouping that implements Deref<Target = HashMap<String, HashSet<String>>>.

Methods

add(&mut self, mail: impl AsRef<str>) -> bool

Adds an email address with automatic validation and normalization using xmail::norm_user_host. Returns true if the email was valid and added successfully, false otherwise. Invalid emails are logged and ignored.

user_li(&self, host: &str) -> Option<&HashSet<String>>

Returns the set of users for a given host domain, or None if the domain doesn't exist.

Trait Implementations

Deref<Target = HashMap<String, HashSet<String>>>

Allows direct read-only access to underlying HashMap methods like len(), is_empty(), iter(), get(), etc.

DerefMut<Target = HashMap<String, HashSet<String>>>

Provides mutable access to the underlying HashMap, enabling direct manipulation using methods like entry(), insert(), remove(), etc. This allows for advanced operations while maintaining type safety.

FromIterator<T: AsRef<str>>

Enables creation from any iterator of string-like items using from_iter(). Each item is validated and added automatically, making it easy to create HostUserLi from collections of email addresses.

Methods

new(sender: impl AsRef<str>, to_li: impl IntoIterator<Item = impl AsRef<str>>, body: impl Into<Vec<u8>>) -> Option<Self>

Creates a new Mail instance with automatic email processing:

• Sender Validation: Uses xmail::norm_user_host to validate and normalize the sender address.
• Recipient Processing: Uses HostUserLi::from_iter() to process all recipients in a functional style.
• Automatic Validation: Each recipient is validated and normalized using xmail::norm_user_host.
• Filtering: Invalid email addresses are automatically filtered out and logged.
• Deduplication: Recipients are automatically deduplicated using HashSet within each domain.
• Grouping: Recipients are organized by domain (host) for efficient processing.
• Return: Returns None if no valid recipients remain after filtering, or if the sender is invalid.

Trait Implementations

IntoIterator for &'a Mail (requires send feature)

Implements IntoIterator for &Mail, returning a custom MailIter<'a> iterator.

• Domain Grouping: Automatically groups recipients by email domain, with each MailMessage containing all recipients for the same domain.
• Lazy Construction: The iterator lazily creates MailMessage instances during iteration, avoiding pre-allocation of a Vec.
• Efficient Delivery: Multiple recipients in the same domain can be delivered in a single SMTP transaction.

struct UserMail

A wrapper around Mail associating it with a user ID.

• mail: Mail: The email content.
• user_id: u64: The unique identifier of the user associated with this mail.

struct MailMessage<'a> (requires send feature)

Represents an email grouped by recipient domain.

• sender_user: &'a str: The user part of the sender's email address.
• sender_host: &'a str: The domain part of the sender's email address.
• domain: &'a str: The recipient domain name (e.g., "gmail.com").
• to_li: Vec<Address<'a>>: List of all recipient addresses for this domain.
• body: &'a [u8]: The email body content.

Trait Implementations

IntoMessage<'a>

Implements the mail_send::smtp::message::IntoMessage trait, allowing MailMessage to be directly converted into an SMTP message containing all recipients. This enables MailMessage to be passed directly to client.send() for batch sending.

Also implements IntoMessage for &MailMessage, allowing message creation without consuming ownership.

IntoIterator

Implements the IntoIterator trait, returning a custom MailMessageIter iterator.

• Zero Allocation: The iterator lazily constructs Message instances during iteration, avoiding the overhead of creating an intermediate Vec.
• Fallback Strategy: Converts MailMessage into multiple individual Message instances (each containing one recipient). This is useful when batch sending fails, allowing for individual delivery to ensure valid recipients receive the email.

Historical Context

RFC 822 and the Separation of Envelope and Content

The design of email systems dates back to the early 1980s with the publication of RFC 822 (Standard for the Format of ARPA Internet Text Messages) and RFC 821 (Simple Mail Transfer Protocol). A key architectural decision was the separation of the "envelope" (handled by SMTP for routing) from the "content" (the message headers and body defined by RFC 822).

mail_struct honors this tradition by focusing on the structure of the message (the content), while delegating the transport (the envelope and transmission) to specialized libraries like mail-send. This modular approach mirrors the original design philosophy of the internet's most enduring communication protocol, ensuring flexibility and maintainability.

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About

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We are redefining the development paradigm of the Internet in a componentized way. Welcome to follow us:

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mail_struct : 极简 Rust 邮件结构库

mail_struct 是一个轻量级的 Rust 库，旨在为邮件消息定义清晰且高效的结构。它提供了与 bitcode（用于高效编码/解码）和 mail-send（用于基于域名分组的 SMTP 传输）的可选集成，使其成为 Rust 应用中处理邮件的灵活选择。

目录

• 功能特性
• 使用指南
• 设计理念
• 技术栈
• 目录结构
• API 文档
• 历史背景

功能特性

• 核心结构: 定义了 Mail、UserMail 和 HostUserLi 结构体来表示邮件数据，提供类型安全的域名分组。
• 序列化: 通过 encode 和 decode 特性支持使用 bitcode 进行高性能的二进制序列化。
• SMTP 集成: 可选的 send 特性支持按域名分组收件人，实现高效的邮件投递。
• 类型安全: 利用 Rust 的类型系统和自定义类型如 HostUserLi 确保数据完整性并防止误用。

使用指南

在 Cargo.toml 中添加 mail_struct：

[dependencies]
mail_struct = { version = "0.1.18", features = ["send", "encode", "decode"] }

创建并按域名分组邮件

use mail_struct::Mail;

#[cfg(feature = "send")]
async fn example() {
    // Mail::new() 会自动：
    // - 使用 xmail::norm_user_host 规范化和验证邮箱地址
    // - 过滤掉无效的邮箱地址
    // - 对收件人进行去重
    // 返回 Option<Mail>，如果没有有效收件人则返回 None
    let mail = Mail::new(
        "sender@example.com",
        vec![
            "user1@gmail.com",
            "user2@yahoo.com",
            "user3@gmail.com",
            "user1@gmail.com",  // 重复 - 会被去除
            "invalid-email",     // 无效 - 会被过滤
        ],
        b"Hello, this is a test email!",
    ).unwrap(); // 生产环境中请处理 None

    // 直接迭代 &mail 即可按域名分组获取 MailMessage
    for msg in &mail {
        println!("Sending to domain: {}", msg.domain);
        
        // 1. 尝试批量发送给该域名的所有收件人
        // MailMessage 实现了 IntoMessage trait，可以直接传给 client.send()
        if let Err(e) = client.send(&msg).await {
            println!("✗ 批量发送失败: {}，开始逐个投递", e);
            
            // 2. 如果批量发送失败，利用 IntoIterator 逐个发送
            // 使用自定义迭代器，零内存分配
            for individual_message in msg {
                match client.send(individual_message).await {
                    Ok(_) => println!("  ✓ 单个邮件发送成功"),
                    Err(e) => println!("  ✗ 单个邮件发送失败: {}", e),
                }
            }
        } else {
             println!("✓ 批量发送成功");
        }
    }
}

设计理念

本库遵循关注点分离原则。核心 lib.rs 定义了数据结构（Mail, UserMail），保持基础依赖最小化。序列化和发送等功能通过特性标志（encode, decode, send）进行门控，允许用户仅按需开启。

当 send 特性激活时，send.rs 模块为 &Mail 实现了 IntoIterator，返回按域名分组的 MailMessage。这种优化减少了所需的 SMTP 连接数量并提高了投递效率。同时，MailMessage 也实现了 IntoIterator，提供了零开销的降级方案，确保在部分收件人地址错误导致批量发送失败时，仍能逐个投递给其他有效的收件人。

技术栈

• Rust: 核心开发语言。
• xmail: 邮箱验证和规范化。
• bitcode (可选): 用于快速二进制编码和解码。
• mail-send (可选): 用于构建和发送 SMTP 消息。

目录结构

.
├── Cargo.toml          # 项目配置
├── README.md           # 主文档
├── readme              # 多语言文档
│   ├── en.md           # 英文 README
│   └── zh.md           # 中文 README
├── src
│   ├── lib.rs          # 核心结构定义及特性门控
│   ├── host_user_li.rs # HostUserLi 类型，用于基于域名的收件人分组
│   └── send.rs         # SMTP 消息逻辑 (特性: send)
└── tests
    └── main.rs         # 集成测试

API 文档

struct Mail

表示一个基础的邮件消息。

• sender_user: String: 发件人邮箱地址的用户部分。
• sender_host: String: 发件人邮箱地址的域名部分。
• host_user_li: HostUserLi: 使用类型安全包装器对 HashMap<String, HashSet<String>> 进行域名分组的收件人。
• body: Vec<u8>: 邮件的原始正文内容。

struct HostUserLi

基于域名的收件人分组的类型安全包装器，实现了 Deref<Target = HashMap<String, HashSet<String>>>。

方法

add(&mut self, mail: impl AsRef<str>) -> bool

添加邮箱地址并自动使用 xmail::norm_user_host 进行验证和规范化。如果邮箱有效并成功添加则返回 true，否则返回 false。无效邮箱会被记录日志并忽略。

user_li(&self, host: &str) -> Option<&HashSet<String>>

返回给定主机域名的用户集合，如果域名不存在则返回 None。

Trait 实现

Deref<Target = HashMap<String, HashSet<String>>>

允许直接只读访问底层 HashMap 的方法，如 len()、is_empty()、iter()、get() 等。

DerefMut<Target = HashMap<String, HashSet<String>>>

提供对底层 HashMap 的可变访问，支持使用 entry()、insert()、remove() 等方法进行直接操作。这允许进行高级操作的同时保持类型安全。

FromIterator<T: AsRef<str>>

支持使用 from_iter() 从任何字符串类型的迭代器创建实例。每个项目都会自动验证和添加，便于从邮箱地址集合创建 HostUserLi。

方法

new(sender: impl AsRef<str>, to_li: impl IntoIterator<Item = impl AsRef<str>>, body: impl Into<Vec<u8>>) -> Option<Self>

创建新的 Mail 实例，自动处理邮箱：

• 发件人验证：使用 xmail::norm_user_host 验证和规范化发件人地址。
• 收件人处理：使用 HostUserLi::from_iter() 以函数式风格处理所有收件人。
• 自动验证：每个收件人都使用 xmail::norm_user_host 进行验证和规范化。
• 过滤：无效的邮箱地址会自动过滤并记录日志。
• 去重：在每个域名内使用 HashSet 自动去除重复的收件人。
• 分组：按域名（主机）组织收件人，提高处理效率。
• 返回：如果过滤后没有有效收件人或发件人无效，则返回 None。

Trait 实现

IntoIterator for &'a Mail (需要 send 特性)

为 &Mail 实现了 IntoIterator，返回自定义的 MailIter<'a> 迭代器。

• 按域名分组：自动按邮件域名分组收件人，每个 MailMessage 包含同一域名的所有收件人。
• 惰性构造：迭代器在遍历时惰性创建 MailMessage 实例，避免预先分配 Vec。
• 高效投递：同一域名的多个收件人可以在单个 SMTP 事务中投递。

struct UserMail

Mail 的包装器，将其与用户 ID 关联。

• mail: Mail: 邮件内容。
• user_id: u64: 与此邮件关联的用户的唯一标识符。

struct MailMessage<'a> (需要 send 特性)

表示按收件人域名分组的邮件。

• sender_user: &'a str: 发件人邮箱地址的用户部分。
• sender_host: &'a str: 发件人邮箱地址的域名部分。
• domain: &'a str: 收件人域名（例如 "gmail.com"）。
• to_li: Vec<Address<'a>>: 该域名下的所有收件人地址列表。
• body: &'a [u8]: 邮件正文内容。

Trait 实现

IntoMessage<'a>

实现了 mail_send::smtp::message::IntoMessage trait，允许 MailMessage 直接转换为包含所有收件人的 SMTP 消息。这使得 MailMessage 可以直接传递给 client.send() 进行批量发送。

同时为 &MailMessage 实现了 IntoMessage，允许在不消耗所有权的情况下创建消息。

IntoIterator

实现了 IntoIterator trait，返回自定义的 MailMessageIter 迭代器。

• 零分配：迭代器在遍历时惰性构造 Message，避免了创建中间 Vec 的开销。
• 降级策略：将 MailMessage 转换为多个独立的 Message（每个包含一个收件人）。这在批量发送失败时非常有用，可以逐个投递以确保有效地址的收件人能收到邮件。

历史背景

RFC 822 与信封/内容的分离

电子邮件系统的设计可以追溯到 20 世纪 80 年代初 RFC 822（ARPA 互联网文本消息格式标准）和 RFC 821（简单邮件传输协议）的发布。一个关键的架构决策是将"信封"（由 SMTP 处理用于路由）与"内容"（由 RFC 822 定义的消息头和正文）分离开来。

mail_struct 秉承了这一传统，专注于消息的结构（内容），而将传输（信封和发送）委托给像 mail-send 这样的专用库。这种模块化的方法反映了互联网最持久通信协议的原始设计哲学，确保了灵活性和可维护性。

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