go-mail

go-mail is a comprehensive, idiomatic Go library for composing and delivering email. It combines mail message construction, MIME encoding, DKIM and S/MIME signing, and SMTP delivery into a single package built almost entirely on the Go standard library and its official extended packages, giving Go programs the capabilities of a full mail user agent without pulling in a heavy dependency tree.

Under the hood it forks and extends Go’s net/smtp package with additional authentication mechanisms (including NTLM and SCRAM-SHA-256-PLUS), concurrency-safe connection reuse, and a pluggable error registry for non-RFC-conformant servers. It supports attachments and inline embeds from files, io.Reader, or embed.FS, HTML/text templating, middleware for altering messages before send, and EML import/export for interoperability with other mail tools.

What You Get

• A functional-options Client/Msg API for building and sending mail without constructor boilerplate
• Support for 7 SMTP authentication mechanisms including OAuth2 (XOAUTH2), NTLM, and SCRAM-SHA-256-PLUS
• Built-in DKIM and S/MIME (RSA and ECDSA) message signing
• Attachment and inline-embed support from the filesystem, io.Reader, embed.FS, or fs.FS
• HTML and text templating support via Go’s html/template and text/template
• EML file import/export for interoperability with mail user agents

Common Use Cases

• Sending transactional email (password resets, notifications, receipts) from a Go backend
• Delivering DKIM- and S/MIME-signed mail for organizations with strict deliverability/compliance requirements
• Composing and sending templated HTML emails with inline images and attachments
• Building internal tooling or CLIs that need to send mail via SMTP, sendmail, or a UNIX domain socket
• Testing mail delivery code with output-to-file (.eml) support instead of a live SMTP server

Under The Hood

Architecture The root mail package is built around three primary types — Client (client.go, functional-options constructor NewClient(host, opts...)), Msg (msg.go, functional-options NewMsg(opts...), holding headers, parts, and attachments), and MsgWriter (msgwriter.go, an RFC 2047/2045-aware multipart serializer with explicit header/body line-length constants). Delivery flows Msg → MsgWriter → the vendored smtp package (a fork of Go’s net/smtp), with cross-cutting concerns pulled into internal packages: internal/dkim for DKIM signing, internal/pkcs7 plus root smime.go for S/MIME signing, internal/ntlm for NTLM auth, and internal/helper for shared utilities. A Middleware interface lets third-party code transform a Msg before send, and quicksend.go’s QuickSend() composes Client and Msg into a one-shot helper for simple cases. The layering is clean and modular; the one architecturally load-bearing decision is forking net/smtp rather than wrapping it, which means any future stdlib SMTP changes must be re-ported by hand.

Tech Stack go-mail targets Go 1.25 and depends on only two modules beyond the standard library: golang.org/x/crypto (S/MIME and DKIM cryptography) and golang.org/x/text (charset/encoding handling) — an intentionally minimal footprint built on crypto/tls, mime/multipart, net/textproto, and context. It ships a Nix flake (flake.nix/flake.lock) alongside standard go build/go test tooling, with GitHub Actions CI, CodeQL static analysis, and OpenSSF Scorecard checks, plus Codecov coverage reporting and REUSE/SPDX license-header compliance across the source tree.

Code Quality The repository carries 19 _test.go files sitting alongside their corresponding implementation files (auth_test.go, client_test.go, dkim_test.go, msg_test.go, smtp/smtp_test.go, and more) using Go’s standard testing package with table-driven patterns, and codecov.yml shows coverage is tracked and gated in CI. Errors are explicit and typed rather than swallowed — senderror.go defines custom delivery error types, and functions consistently wrap errors with %w for traceability. Naming follows idiomatic Go conventions (With* functional options, Set*/Get* accessors), and every exported symbol carries dense, consistently formatted GoDoc comments describing parameters and return values. CodeQL and OpenSSF Scorecard both run on every change in addition to the test suite.

What Makes It Unique Most Go SMTP libraries either wrap the unmodified standard-library net/smtp package (missing OAuth2, NTLM, and SCRAM support) or shell out to an external sendmail binary. go-mail instead forks and extends net/smtp directly, adding concurrency-safe connection reuse, a pluggable error registry for non-RFC-conformant servers, and seven distinct SMTP authentication mechanisms — while also layering on DKIM signing, S/MIME (RSA and ECDSA) signing, DSN/MDN request headers, and EML import/export, all on top of the standard library plus two extended packages rather than a C library binding. This is deep, deliberate engineering investment in a narrow domain rather than a novel architectural paradigm.