Intro

Using email to send notifications seems old school nowadays. Quite often its nonetheless a tool that is available and accepted in many settings and where recipients are already familiar using it.

While sending emails from Linux machines is quite easy, sending them from Windows always has been is a little bit tricky. Windows neither ships with with SMTP servers (Simple Mail Transfer Protocol) nor has it a tradition of using package managers for easily and reliably installing additional software and services.

There are several solutions to to sending emails on Windows. Most of them involve connecting to a remote SMTP server or API some others wrap around dedicated programs like e.g. blat. This blog post on mailtrap.io lists a lot of possibilities (also here, and here).

Sending Emails with {curl} and Docker

Now, let us add another way which I find has some nice properties. Why?

• Because it allows to send emails directly from my local computer.
• It allows for clean installation and removal of software.
• With a little effort the whole process can be scripted meaning that installation is automatically transparent and automated.
• It’s easy to switch email backends as well as to port it to Linux machines e.g. to put into production.
• It does not need any credentials to work.

What do we need?

• The {curl} package to handle the SMTP protocol.
• Docker to provide a container infrastructure on which to run Linux distributions for a painless installation experience.
• The smtp DOCKERFILE by bytemark, e.g. (see here, and here) providing the recipe to install and run a SMTP server as Docker container – in this case exim.

Now, on the commandline we can ask docker to retrieve everything to create a image (docker create) for us based on bytemark/smtp and name it mail (--name mail). To ensure we have excess to the SMTP server within the container we furthermore ask Docker to expose port 25 such that it can only be accessed locally -p 127.0.0.1:25:25.

docker create --name mail -p 127.0.0.1:25:25 bytemark/smtp 

With the following we can start a container.

docker start mail

This let’s us get an overview over running containers.

docker container ls

This allows for first stopping and than deleting the container.

docker stop mail

When the container is running we can use {curl} to send emails from our local computer via the instant SMTP server to whatever email recipient.

library(curl)

send_mail(
    mail_from = "[email protected]", 
    mail_rcpt = "[email protected]", 
    message   = "Subject: Welcome\nGreetings from Windows.\n"
)
## $url
## [1] "smtp://localhost"
## 
## $status_code
## [1] 250
## 
## $type
## [1] NA
## 
## $headers
##   [1] 32 35 30 20 4f 4b 0d 0a 32 35 30 20 41 63 63 65 70 74 65 64 0d 0a 33 35 34 20 45 6e 74 65 72 20 6d 65 73 73 61 67 65 2c
##  [41] 20 65 6e 64 69 6e 67 20 77 69 74 68 20 22 2e 22 20 6f 6e 20 61 20 6c 69 6e 65 20 62 79 20 69 74 73 65 6c 66 0d 0a 32 35
##  [81] 30 20 4f 4b 20 69 64 3d 31 6b 46 49 35 7a 2d 30 30 30 30 33 72 2d 4c 62 0d 0a
## 
## $modified
## [1] NA
## 
## $times
##      redirect    namelookup       connect   pretransfer starttransfer         total 
##      0.000000      0.000066      0.000069      0.000000      0.004420      0.012461 
## 
## $content
## raw(0)

Caveats

One draw back of all solutions not using a proper email providers is that messages most likely will be flagged as spam. Well, sending emails from some random IP address is rather suspicious – don’t you think – it literally could be anyone doing this?!