Stalwart mail-parser

Stalwart mail-parser is a Rust library for reading email messages and their MIME contents. It suits developers building Rust software that needs to inspect incoming mail; it is a poor fit when the job is to compose messages or use a hosted parsing service.

Overview

The library handles Internet Message Format, MIME and related messaging standards, while trying to recover messages that are imperfect but still close enough to those standards. Its useful distinction is control: parsing stays inside a Rust application, with message parts and content exposed for the application to process.

That focus also defines its boundary. mail-parser reads messages; it does not create them. The project points to the separate mail-builder crate for message construction.

Key features

MIME content and attachments

Parsing produces a MIME tree plus lists of text bodies, HTML bodies and attachments. When one text format is absent, the library can convert HTML to plain text or plain text to HTML. That gives applications a practical fallback for displaying a message, though the conversion does not replace a message-building workflow.

Parsing, speed and memory

The parser aims to tolerate non-conforming messages when they do not stray too far from the standard. SIMD kernels accelerate boundary and header scanning on supported targets, and transfer decoding and charset conversion are deferred until content is read. This design can avoid doing work for content an application never consumes. The README says practically all returned strings reference the raw input message, a useful memory characteristic for applications retaining parsed data alongside the original message.

Charsets, limits and integration

It recognizes 41 character sets using WHATWG encoding labels. The optional full_encoding feature adds legacy multibyte support, including Shift_JIS, Big5 and GB18030. No Cargo features are enabled by default; the optional serde feature adds serialization for Message and related types.

Default limits allow nesting to 64 levels, up to 10,000 MIME parts and up to three levels of decoded nested messages. Those caps provide concrete boundaries for parsing complex input. The project says every library function has been fuzzed and tested with MIRI, and that Stalwart Mail Server uses the library in production. Neither claim removes the need for an application to handle its own parsing failures and input policy.

Pricing

Both named plans cost nothing: Open-source library and mail-parser. The library is free and open source under either the MIT License or Apache License 2.0, at the user's option; there is no paid plan or usage limit stated on the repository page. There is no free trial because the software is already free. These are library plans, not hosted-service tiers with monthly email quotas.

Platforms

Stalwart mail-parser is listed for Linux, macOS and self-hosted use. It requires Rust 1.98 or later, so projects on older toolchains cannot adopt it without upgrading. The minimum Rust version and its library form make it a developer dependency rather than a ready-to-use mail service.

Who it's for

Choose it for Rust applications that need standards-aware parsing, MIME body and attachment access, and control over when content is decoded. Its permissive dual-license choice and lack of stated usage limits make it a straightforward option to evaluate for self-hosted software. Look elsewhere if you need to build outbound email, run parsing through a web or API service, or support a Rust toolchain below 1.98.

Pros and cons

• Pros: MIME tree, text and HTML body lists, and attachment parsing cover the main components of incoming email.
• Pros: Lazy decoding and input-referencing strings can reduce unnecessary work and memory duplication.
• Pros: Fuzzing and MIRI testing across library functions, plus production use in Stalwart Mail Server, are meaningful engineering signals.
• Cons: It does not build messages, so applications that send mail need another crate.
• Cons: Legacy multibyte charset support requires opting into full_encoding; features are not enabled by default.
• Cons: Rust 1.98 is a firm prerequisite, limiting adoption in projects pinned to an older compiler.

Alternatives

Email Parsing Software is the broader category directory for comparing other options.

• PHP MIME Mail Parser is the closer choice for a PHP application: it is a free, MIT-licensed open-source library for PHP 8.2+ and requires the mailparse extension.
• CloudMailin fits teams seeking a hosted API or web service rather than a Rust dependency. Its free tier includes 10,000 emails per month with a 512KB per-message cap; Starter is $25 a month for 10,000 emails per month and 2MB per message.
• MailFrame is another API/web option, with a free tier of 100 parse operations per month and one active schema, or an Indie plan at $19 a month.
• noima offers API and web parsing with a free allowance of 10 documents once and 30 days' retention; its Starter plan is €12 a month for 150 documents per month and 60 days' retention.
• Parserr is an API/web alternative whose free tier covers 15 emails per month, 10 documents per month and one inbox, with parsing rules and integrations included.
• Parsio Receipt OCR is an API/web alternative with a free tier of 30 credits per month, 30-day data retention and one user.
• Parsli is an API/web alternative with 10 free pages on signup, unlimited parsers and API access plus integrations.
• DigiParser is an API/web alternative with a free tier described as 100 documents.

Verdict

For developers building Rust software that must parse incoming email locally, Stalwart mail-parser is a strong, focused choice: it combines MIME and attachment access with lazy decoding and no stated usage caps, at no cost. Choose something else if you need message composition, a hosted parsing workflow, or compatibility with a Rust compiler earlier than 1.98.