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How to Send Email with Nim

Nim offers an smtp module, but it is a bit annoying to use out of the box. This blogpost hopes to be a mini-tutorial on the basics of how to use the smtp library and give developers best practices for handling outgoing email in ways that Google or iCloud will accept.

SMTP in a Nutshell

SMTP, or the Simple Mail Transfer Protocol is the backbone of how email works. It’s a very simple line-based protocol, and there are wrappers for it in almost every programming language. Usage is pretty simple:

Unfortunately, the devil is truly in the details here. There are a few things that absolutely must be present in your emails in order for services like GMail to accept them. They are:

For a more complete example, let’s create a Mailer type and a constructor:

# mailer.nim
import asyncdispatch, logging, smtp, strformat, strutils

type Mailer* = object
  address: string
  port: Port
  myAddress: string
  myName: string
  username: string
  password: string
proc newMailer*(address, port, myAddress, myName, username, password: string): Mailer =
  result = Mailer(
    address: address,
    port: port.parseInt.Port,
    myAddress: myAddress,
    myName: myName,
    username: username,
    password: password,

And let’s write a mail method to send out email:

proc mail(m: Mailer, to, toName, subject, body: string) {.async.} =
    toList = @[fmt"{toName} <{to}>"]
    msg = createMessage(subject, body, toList, @[], [
      ("From", fmt"{m.myName} <{m.myAddress}"),
      ("MIME-Version", "1.0"),
      ("Content-Type", "text/plain"),

  var client = newAsyncSmtp(useSsl = true)
  await client.connect(m.address, m.port)
  await client.auth(m.username, m.password)
  await client.sendMail(m.myAddress, toList, $msg)
  info "sent email to: ", to, " about: ", subject
  await client.close()

Breaking this down, you can clearly see the parts of the SMTP connection as I laid out before. The Mailer creates a new transient SMTP connection, authenticates with the remote server, sends the properly formatted email to the server and then closes the connection cleanly.

If you want to test this code, I suggest testing it with a freely available email provider that offers TLS/SSL-encrypted SMTP support. This also means that you need to compile this code with --define: ssl, so create config.nims and add the following:

--define: ssl

Here’s a little wrapper using cligen:

when isMailModule:
  import cligen, os
    smtpAddress = getEnv("SMTP_ADDRESS")
    smtpPort = getEnv("SMTP_PORT")
    smtpMyAddress = getEnv("SMTP_MY_ADDRESS")
    smtpMyName = getEnv("SMTP_MY_NAME")
    smtpUsername = getEnv("SMTP_USERNAME")
    smtpPassword = getEnv("SMTP_PASSWORD")
  proc sendAnEmail(to, toName, subject, body: string) =
    let m = newMailer(smtpAddress, smtpPort, smtpMyAddress, smtpMyName, smtpUsername, smtpPassword)
    waitFor m.mail(to, toName, subject, body)

Usage is simple:

$ nim c -r mailer.nim --help
  sendAnEmail [required&optional-params]
Options(opt-arg sep :|=|spc):
  -h, --help                         print this cligen-erated help
  --help-syntax                      advanced: prepend,plurals,..
  -t=, --to=       string  REQUIRED  set to
  --toName=        string  REQUIRED  set toName
  -s=, --subject=  string  REQUIRED  set subject
  -b=, --body=     string  REQUIRED  set body

I hope this helps, this module is going to be used in my future post on how to create an application using Nim’s Jester framework.

This article was posted on 2019-08-28. Facts and circumstances may have changed since publication. Please contact me before jumping to conclusions if something seems wrong or unclear.

Series: howto

Tags: #nim #email