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Internet email library for Clojure

postal is a library for constructing and sending RFC822-compliant Internet email messages. It wraps the Jakarta Mail package for message and SMTP support. It supports sendmail natively. Supports STARTTLS & SSL.


  • Anything that can run Java should work
  • sendmail support likely requires *nix, but msmtp, et al., are worthy substitutes
  • Untested on Windows


  • Jakarta Mail (in lib/ after build)
  • Java 8+


Served by Clojars. In your Leiningen project.clj:

Clojars Project

Likewise substitute any tag name from git.



At a bare minimum, provide a map with :from and :to (and you'll probably also be wanting :subject and :body, though they're technically optional). Any other keys you supply will show up as ancillary headers. This example will locally inject the message into whatever sendmail-compatible interface your system provides.

user> (in-ns 'postal.core)
#<Namespace postal.core>
postal.core> (send-message {:from ""
                            :to ["" ""]
                            :cc ""
                            :subject "Hi!"
                            :body "Test."
                            :X-Tra "Something else"})
{:code 0, :error :SUCCESS, :message "message sent"}


To use SMTP, add an argument map before the message with at least :host key.

postal.core> (send-message {:host ""}
                           {:from ""
                            :to ""
                            :subject "Hi!"
                            :body "Test."})
{:code 0, :error :SUCCESS, :message "message sent"}

For legacy compatibility, you can also supply these connection parameters as metadata on the message. (send-message ^{:host ...} {:from ...})


Authenticate to SMTP server with :user and :pass.

postal.core> (send-message {:host ""
                            :user "jsmith"
                            :pass "sekrat!!1"}
                           {:from ""
                            :to ""
                            :subject "Hi!"
                            :body "Test."})
{:code 0, :error :SUCCESS, :message "message sent"}

Encryption (Gmail example)

You probably do not want to do this in the clear, so add :ssl to get an encrypted connection. This will default to port 465 if you don't specify one.

If your destination supports TLS instead, you can use :tls. This will default to port 25, however, so if you need a different one make sure you supply :port. (It's common for ISPs to block outgoing port 25 to relays that aren't theirs.)

postal.core> (send-message {:host ""
                            :user ""
                            :pass "sekrat!!1"
                            :port 587
                            :tls true}
                           {:from ""
                            :to ""
                            :subject "Hi!"
                            :body "Test."})
{:code 0, :error :SUCCESS, :message "message sent"}

(Despite the documentation mentioning SSL, your Google account is most likely restricted to TLS on port 587 and furthermore you may have to configure your account to "allow less secure apps" in order to send emails.)


Since Amazon SES uses authenticated SMTP, postal can use it. Just make sure you use a verified address and your SMTP credentials (visit the AWS Console to set those up). Also, if you're just sandboxing, you can only send to a verified address as well. Example:

postal.core> (send-message {:user "AKIAIDTP........" :pass "AikCFhx1P......."
                            :host ""
                            :port 587}
               {:from "" :to ""
                :subject "Test from Amazon SES" :body "Test!!!11"})
{:error :SUCCESS, :code 0, :message "messages sent"}


Attachments and multipart messages can be added as sequences of maps:

postal.core> (send-message {:host ""}
                           {:from ""
                            :to ""
                            :subject "Hi!"
                            :body [{:type "text/html"
                                    :content "<b>Test!</b>"}
                                   ;;;; supports both dispositions:
                                   {:type :attachment
                                    :content ( "/tmp/foo.txt")}
                                   {:type :inline
                                    :content ( "/tmp/a.pdf")
                                    :content-type "application/pdf"}]})
{:code 0, :error :SUCCESS, :message "message sent"}

If your attachment has a content-type that is not recognized by Jakarta Mail, e.g., .pdf or .doc, you can set :content-type. You can also set :file-name and :description if you don't like the filename that :content uses.

If you want another multipart type than "mixed", you can specify it as a keyword as the first value in the map sequence. That way you can for example create an HTML-Email that displays a text message as fallback in email clients that do not support (or suppress) HTML-mails:

postal.core> (send-message {:host "localhost"
                            :port 2500
                            :user "user@localhost"
                            :pass "somePassword"}
                       {:from ""
                        :to ""
                        :subject "multipart/alternative test"
                        :body [:alternative
                               {:type "text/plain"
                                :content "This is a test."}
                               {:type "text/html"
                                :content "<html><head> </head><body>
                                <h1>Heading 1</h1><p>This is a test.</p>


Postal uses Jakarta Mail underneath, which defaults to charset us-ascii. To set the charset, set the :type, like "text/html; charset=utf-8".

Message ID

Postal will supply a message ID by default that looks like [random]@postal.[host]. You can customize this by supplying a :message-id header with a function that takes no args. The included can be used if you'd like to make use of its randomness and only customize the hostname.

{:from "foo@bar.dom"
 :to "baz@bar.dom"
 :subject "Message IDs!"
 :body "Regards."
 :message-id #( "")}

User Agent

You can customize the default User-Agent header (by default postal/VERSION).

{:from "foo@bar.dom"
 :to "baz@bar.dom"
 :subject "Message IDs!"
 :body "Regards."
 :user-agent "MyMailer 1.0"}


You can stress-test a server by:

postal.core> (stress ^{:host "localhost"
                       :num     1000
                       :delay   250   ;; msecs
                       :threads 5     ;; concurrent connections}
                     {:from "foo@lolz.dom"
                      :to "bar@lolz.dom"})
sent 1000 msgs to localhost:25


% lein deps && lein jar


Alexander Kouznetsov
Allen Rohner
Andre Branco
Andres Cuervo
Andy Fingerhut
Camille Troillard
Christoph Henkelmann
Colin Jones
Dante Briones
Dimas Guardado
Gerrit Hentschel
J. David Lowe
Jeff Palmucci
Joe Gallo
Kevin DeJong
Kyle Kingsbury
Paul Biggar
Paul Stadig
Phil Hagelberg
Peter Onneby
Roman Flammer
Sam Ritchie
Stephen Nelson


Postal is (c) 2009-2019 Andrew A. Raines and released under the MIT license.

Can you improve this documentation? These fine people already did:
Drew Raines, Andrew Raines, Allen Rohner, Christoph Henkelmann, Joe Gallo, Peter Onneby, Gerrit Hentschel & Paul Biggar
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