Stalwart mail-parser

Stalwart mail-parser is a Rust library for extracting structure and content from email messages. It is best suited to developers building Rust software that needs to inspect incoming mail; its focused parsing role makes it a less complete fit for broader mail workflows.

Overview

The library handles Internet Message Format (RFC 5322), MIME (RFC 2045–2049) and related internet messaging standards. It tries to recover messages that depart from those standards without straying too far, useful when real-world mail is imperfect but not a substitute for handling arbitrary malformed input.

Parsing happens within the Rust application, producing structured message parts for the application to use. The boundary is clear: mail-parser reads messages but does not build them; the project points to the separate mail-builder crate for composition.

Key features

MIME bodies and attachments

The parser exposes a MIME tree and lists of text bodies, HTML bodies and attachments. It can convert HTML to plain text, or plain text to HTML, when the other format is absent. That gives an application a fallback representation for messages that provide only one body format, while leaving presentation decisions to the application.

Parsing performance and memory

On supported targets, SIMD kernels scan boundaries and headers. Transfer decoding and character conversion are delayed until content is read, and nearly all returned strings reference the raw input message. Together, these choices are aimed at limiting unnecessary work and copies; the benefit is most relevant to applications processing messages without needing every body immediately.

Character sets and optional capabilities

The library recognizes 41 character sets and WHATWG encoding labels. Enabling the optional full_encoding feature adds decoding for legacy multibyte sets including Shift_JIS, Big5 and GB18030. No Cargo feature is enabled by default, so projects must opt into extra capabilities such as full_encoding or serde, which adds serialization support for Message and related types.

Limits and security testing

Default parser limits include a nesting depth of 64, up to 10,000 MIME parts and up to three levels of decoded nested messages. Those limits provide defined bounds for complex input, but applications with messages beyond them should account for the defaults. The project says every library function has been fuzzed and tested with MIRI, and that Stalwart Mail Server uses the library in production worldwide.

Pricing

mail-parser is free and open source, with the choice of MIT or Apache License 2.0. The repository gives two free plan labels: Open-source library at no cost, and mail-parser at no cost. No paid plan or usage limits are stated on the repository page. There is no free trial; the relevant cost is the engineering work of integrating a Rust library.

Platforms

The listed platforms are Linux, macOS and self-hosted. The minimum supported Rust version is 1.98, a material compatibility requirement for teams maintaining older Rust toolchains.

Who it's for

This is a strong fit for developers building Rust applications that need message structure, body extraction or attachment parsing without handing messages to a hosted service. It is less suitable for teams that need a graphical mail-processing workflow, a hosted API, or message composition as part of the same library.

Pros and cons

• Pros: It returns MIME structure, text and HTML bodies, and attachments, with fallback conversion when one body format is absent.
• Pros: Lazy decoding, reference-based strings and SIMD scanning on supported targets are designed to reduce unnecessary processing and memory copies.
• Pros: MIT or Apache-2.0 licensing and no stated usage caps leave deployment and integration choices open.
• Cons: Rust 1.98 is required, which rules it out for projects that cannot meet that toolchain floor.
• Cons: It cannot create email messages, and optional capabilities require explicit feature selection.
• Cons: Parser tolerance has a boundary: nonconforming input is only handled when it remains reasonably close to the standard.

Alternatives

For another self-hosted open-source library, PHP MIME Mail Parser is the more relevant choice when the application uses PHP 8.2 or later and can meet its mailparse extension requirement.

CloudMailin is worth considering when a hosted API or web service is preferable to embedding a parser: its free plan allows 10,000 emails per month, with a 512KB per-message limit and no card requirement, while its Starter plan is $25 a month for 10,000 emails per month and 2MB per message.

DigiParser is an API and web alternative with a free option; its paid plan starts at $14 a month for 100 payslips, with enterprise pricing for 1,000 or more documents per month.

Document Parser offers a web-based option with a free plan limited to one user and 20 documents; its Premium plan is $69.90 a month per user.

MailFrame is another API and web alternative. Its free plan provides 100 parse operations per month, one active schema, webhook delivery and community support, with no credit card required; its Indie plan is $19 a month.

MailParse offers a hosted API and web service, with a free plan for 100 emails per month, webhook delivery, REST polling, one inbound address and community support; its Pro plan is $29 a month for 10,000 emails per month.

noima is an API and web option whose free tier gives 10 documents once, with 30 days of retention; its Starter plan is €12 a month for 150 documents per month and 60 days of retention.

Parserr is another API and web alternative with a free plan for 15 emails and 10 documents per month, one inbox, unlimited parsing rules and integrations, and Zapier and Microsoft Flow support.

Browse Email Parsing Software for more options.

Verdict

Choose mail-parser when you need a Rust-native way to inspect email structure, bodies and attachments under a permissive open-source license. Its parsing focus, lazy decoding and bounded defaults make it a practical fit for application-level mail handling; choose another tool if you need hosted processing, broader workflow features or message creation.