9 releases

0.9.1 Jun 20, 2020
0.9.0 Jun 6, 2020
0.8.2 May 31, 2020
0.8.1 Nov 6, 2018
0.6.1 Aug 22, 2018

#1472 in Network programming

Used in 3 crates (2 directly)


4.5K SLoC

new-tokio-smtp docs License License

Maintenance Status

This crate is currently passively maintained, this means:

  • I will still respond to bugs, and fix them (if this doesn't involve any major rewrites).
  • I will still evaluate and merge PR's. (As long as I don't get flooded and they don't rewrite the whole crate or similar ;-)

Also the maintenance status might go back to actively maintained in the future.


The new-tokio-smtp crate provides an extendible SMTP (Simple Mail Transfer Protocol) implementation using tokio.

This crate provides only SMTP functionality, this means it does neither provides functionality for creating mails, nor for e.g. retrying sending a mail if the receiver was temporary not available.

While it only provides SMTP functionality it is written in a way to make it easy to integrate with higher level libraries. The interoperability is provided through two mechanisms:

  1. SMTP commands are defined in a way which allow library user to define there own commands, all commands provided by this library could theoretically have been implemented in an external library, this includes some of the more special commands like STARTTLS, EHLO and DATA. Moreover a Connection can be converted into a Io instance which provides a number of useful functionalities for easily implementing new commands, e.g. Io.parse_response.

  2. syntactic construct's like e.g. Domain or ClientId can be parsed but also have "unchecked" constructors, this allows libraries which have there own validation to skip redundant validations, e.g. if a mail library might provide a Mailbox type of mail addresses and names, which is guaranteed to be syntactically correct if can implement a simple From/Into impl to cheaply convert it to an Forward-Path. (Alternative they also could implement their own Mail cmd if this has any benefit for them)

  3. provided commands (and syntax constructs) are written in a robust way, allowing for example extensions like SMTPUTF8 to be implemented on it. The only drawback of this is that it trusts that parts created by more higher level libraries are valid, e.g. it won't validate that the mail given to it is actually 7bit ascii or that it does not contain "orphan" '\n' (or '\r') chars. But this is fine as this library is for using smtp to send mails, but not for creating such mails. (Note that while it is trusting it does validate if a command can be used through checking the result from the last EHLO command, i.e. it wont allow you to send a STARTTLS command on a mail server not supporting it)

  4. handling logic errors (i.e. server responded with code 550) separately from more fatal errors like e.g. a broken pipe


extern crate futures;
extern crate tokio;
extern crate new_tokio_smtp;
extern crate vec1;
extern crate rpassword;

use std::io::{stdin, stdout, Write};

use futures::stream::Stream;
use futures::future::lazy;
use new_tokio_smtp::error::GeneralError;
use new_tokio_smtp::{command, Connection, ConnectionConfig, Domain};
use new_tokio_smtp::send_mail::{
    Mail, EncodingRequirement,
    MailAddress, MailEnvelop,

struct Request {
    config: ConnectionConfig<command::auth::Plain>,
    mails: Vec<MailEnvelop>

fn main() {
    let Request { config, mails } = read_request();
    // We only have iter map overhead because we
    // don't have a failable mail encoding step, which normally is required.
    let mails = mails.into_iter().map(|m| -> Result<_, GeneralError> { Ok(m) });

    println!("[now starting tokio]");
    tokio::run(lazy(move || {
        println!("[start connect_send_quit]");
        Connection::connect_send_quit(config, mails)
            //Stream::for_each is design wise broken in futures v0.1
            .then(|result| Ok(result))
            .for_each(|result| {
                if let Err(err) = result {
                    println!("[sending mail failed]: {}", err);
                } else {
                    println!("[successfully send mail]")

fn read_request() -> Request {

    println!("preparing to send mail with ethereal.email");
    let sender = read_email();
    let passwd = read_password();

    // The `from_unchecked` will turn into a `.parse()` in the future.
    let config = ConnectionConfig
            .expect("resolving domain failed")
        .auth(command::auth::Plain::from_username(sender.clone(), passwd)
            .expect("username/password can not contain \\0 bytes"))

    // the from_unchecked normally can be used if we know the address is valid
    // a mail address parser will be added at some point in the future
    let send_to = MailAddress::from_unchecked("invalid@test.test");

    // using string fmt to crate mails IS A
    // REALLY BAD IDEA there are a ton of ways
    // this can go wrong, so don't do this in
    // practice, use some library to crate mails
    let raw_mail = format!(concat!(
        "Date: Thu, 14 Jun 2018 11:22:18 +0000\r\n",
        "From: You <{}>\r\n",
        //ethereal doesn't delivers any mail so it's fine
        "To: Invalid <{}>\r\n",
        "Subject: I am spam?\r\n",
    ), sender.as_str(), send_to.as_str());

    // this normally adapts to a higher level abstraction
    // of mail then this crate provides
    let mail_data = Mail::new(EncodingRequirement::None, raw_mail.to_owned());

    let mail = MailEnvelop::new(sender, vec1![ send_to ], mail_data);

    Request {
        mails: vec![ mail ]

fn read_email() -> MailAddress {
    let stdout = stdout();
    let mut handle = stdout.lock();
    write!(handle, "enter ethereal.email mail address\n[Note mail is not validated in this example]: ")

    let mut line = String::new();
    stdin().read_line(&mut line).unwrap();

fn read_password() -> String {
    rpassword::prompt_password_stdout("password: ").unwrap()


cargo test --features "mock-impl"

Just running cargo test won't work for now, this might be fixed in the future with cargo supporting "for testing only default features" or similar.

Debugging SMTP

If the log (default) feature is enabled and the log level is set to trace then the whole client/server interaction is logged.

Any line the server sends is logged after receiving it and before parsing it and any line the client sends is logged before sending it (before flushing).

The exception is that send mail bodies are not logged. Furthermore for any line the client send starting with "AUTH" everything except the next word will be redacted to prevent logging passwords, access tokens and similar. For example in case of auth plain login only "AUTH PLAIN <redacted>" will be logged.

This still means that when using trace logging following thinks will still be logged:

  • client id
  • server id
  • server greeting message (can contain the client ip or DNS name depending on the server you connect to).
  • sending mail address
  • all receiving mail addresses

Given that trace logging should only be enabled for debugging purpose this isn't a problem even with GDPR. If you still do set it up so that it's not enabled for this crate. E.g. with env_logger it would be something like `RUST_LOG="new_tokio_smtp=warn,trace" to enabled trace logging for all places but smtp. But you can easily run into GDPR incompatibility as this likely would log e.g. IP addresses of connecting clients and similar.

Note that trace logging does imply a performance overhead above just writing to the log as the trace logging is done on a low level where not string but bytes are handled and as such they have to be converted back to an string additional each command line (not mail msg) the client send needs to be checked to see if it's starts with AUTH and needs to be redacted, etc.


The concept of behind the library is explained in the notes/concept.md file.

Usability Helpers

The library provides a number of usability helpers:

  1. chain::chain provides a easy way to chain a number of SMTP commands, sending each command when the previous command in the chain did not fail in any way.

  2. mock_support feature: Extends the Socket abstraction to not only abstract over the socket being either a TcpStream or a TlsStream but also adds another variant which is a boxed MockStream, making the smtp libraries, but also libraries build on top of it more testable.

  3. mock::MockStream (use the features mock-impl) A simple implementation for a MockStream which allows you to test which data was send to it and mock responses for it. (Through it's currently limited to a fixed predefined conversation, if more is needed a custom MockStream impl. has to be used)

  4. future_ext::ResultWithContextExt: Provides a ctx_and_then and ctx_or_else methods making it easier to handle results resolving as Item to an tuple of a context (here the connection) and a Result belonging to an different abstraction level than the futures Error (here a possible CommandError while the future Error is an connection error like e.g. a broken pipe)

Limitations / TODOs

Like mentioned before this library has some limitations as it's meant to only do SMTP and nothing more. Through there are some other limitations, which will be likely to be fixed in future versions:

  1. no mail address parser for send_mail::MailAddress and neither a parser for ForwardPath/ReversePath (they can be constructed using from_unchecked). This will be fixed when I find a library "just" doing mail addresses and doing it right.

  2. no "build-in" support for extended status codes, this is mainly the way because I hadn't had time for this, changing this in a nice build-in way might require some API changes wrt. to the Response type and it should be done before v1.0

  3. The number of provided commands is currently limited to a small but useful subset, commands which would be nice to provide include BDAT and more variations of AUTH (currently provided are PLAIN and simple LOGIN which is enough for most cases but supporting e.g. OAuth2 would be good)

  4. no support for PIPELINING, while most extensions can be implemented using custom commands, this is not true for pipelining. While there exists a concept how pipelining can be implemented without to much API brakeage this is for now not planed due to time limitations.

  5. no stable version (v1.0) for now, as tokio is not stable yet. When tokio becomes stable a stable version should be released, through another one might have to be released at some point if PIPELINING is implemented later one (through in the current concept for implementing it there are little braking changes, except for implementors of custom commands)


Documentation can be viewed on docs.rs.


Licensed under either of

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

Change Log

  • v0.4:

    • renamed from_str_unchecked to from_unchecked
    • Cmd.exec accepts now Io instead of Connection
      • replace CmdFuture with ExecFuture
      • Connection.send_simple_cmd is now Io.exec_simple_cmd
  • v0.5:

    • improved ClientId
      • renamed ClientIdentity to ClientId
      • added hostname() constructor
    • added builder for ConnectionConfig
      • removed old with_ constructors
    • placed all Auth* commands into a auth module (e.g. AuthPlain => auth::Plain)
    • changed feature naming schema
  • v0.6

    • added connection builder for local non secure connections
    • addec constructors for builders on types they build
    • renamed auth::plain::NullCodePoint to auth::plain::NullCodePointError
  • v0.7

    • send_all_mails and connect_send_quit now accept a IntoIterable instead of stream
      • you need to have all values already ready when sending so Stream didn't fit well
      • it also means you can now pass in a Vec or a std::iter:once
    • GeneralError as no longer the PreviousRequestKilledConnection error variant instead a std::io::Error::new(std::io::ErrorKind::NoConnection, "...") is returned which makes it easier to adapt to by libraries using it and fit the semantics as good as previous solution
    • send_all_mails and connect_send_quit now return a stream instead of a future resolving to a stream.
  • v0.7.1

    • updated dependencies, makes sure tokio doesn't produces a deprecation warning as a import moved to a different place in tokio
  • v0.8.0

    • update tokio-tls/native-tls to v0.2.x
    • renamed method creating a builder from build* to builder*
  • v0.8.1

    • SelectCmd and EitherCmd where added
  • v0.8.2

    • Fix bug where the wrong parsing error was emitted for EsmtpValue
    • Now uses rustfmt.
    • Warn but not fail on un-parsable ehlo capability response lines
  • v0.9.0

    • Some small API cleanup.
    • Made log and (default) feature. So if no log implementor is set up the crate doesn't need to be compiled in.
    • Made it configurable if bad EHLO capability response lines should trigger an syntax error or be skipped (potentially logging the bad keyword/value).
    • Added trace level logging to log the whole server conversation except mail bodies and passwords.
  • v0.9.1

    • Trace log to which socket address a smtp connection is established (or is failed to be established).



~150K SLoC