SMTP

SMTP

SMTP stands for Simple Mail Transfer Protocol.

curl supports sending data to a an SMTP server, which combined with the right
set of command line options makes an email get sent to a set of receivers of
your choice.

When sending SMTP with curl, there are a two necessary command line options
that must be used.

You need to tell the server at least one recipient with --mail-rcpt. You
can use this option several times and then curl will tell the server that
all those email addresses should receive the email.
You need to tell the server which email address that is the sender of the
email with --mail-from. It is important to realize that this email
address is not necessarily the same as is shown in the From: line of the
email text.

Then, you need to provide the actual email data. This is a (text) file
formatted according to RFC
5322. It is a set of headers and a
body. Both the headers and the body need to be correctly encoded. The headers
typically include To:, From:, Subject:, Date: etc.

A basic command to send an email:

curl smtp://mail.example.com --mail-from myself@example.com --mail-rcpt
receiver@example.com --upload-file email.txt

Example email.txt

From: John Smith <john@example.com>
To: Joe Smith <smith@example.com>
Subject: an example.com example email
Date: Mon, 7 Nov 2016 08:45:16
Dear Joe,
Welcome to this example email. What a lovely day.

Secure mail transfer

Some mail providers allow or require using SSL for SMTP. They may use a
dedicated port for SSL or allow SSL upgrading over a plaintext connection.

If your mail provider has a dedicated SSL port you can use smtps:// instead of
smtp://, which uses the SMTP SSL port of 465 by default and requires the entire
connection to be SSL. For example smtps://smtp.gmail.com/.

However, if your provider allows upgrading from plaintext to secure transfers
you can use one of these options:

--ssl           Try SSL/TLS (FTP, IMAP, POP3, SMTP)
--ssl-reqd      Require SSL/TLS (FTP, IMAP, POP3, SMTP)

You can tell curl to try but not require upgrading to secure transfers by
adding --ssl to the command:

curl --ssl smtp://mail.example.com --mail-from myself@example.com
     --mail-rcpt receiver@example.com --upload-file email.txt

You can tell curl to require upgrading to using secure transfers by adding
--ssl-reqd to the command:

curl --ssl-reqd smtp://mail.example.com --mail-from myself@example.com
     --mail-rcpt receiver@example.com --upload-file email.txt

The SMTP URL

The path part of a SMTP request specifies the host name to present during
communication with the mail server. If the path is omitted then curl will
attempt to figure out the local computer’s host name and use that. However,
this may not return the fully qualified domain name that is required by some
mail servers and specifying this path allows you to set an alternative name,
such as your machine’s fully qualified domain name, which you might have
obtained from an external function such as gethostname or getaddrinfo.

To connect to the mail server at mail.example.com and send your local
computer’s host name in the HELO / EHLO command:

curl smtp://mail.example.com

You can of course as always use the -v option to get to see the
client-server communication.

To instead have curl send client.example.com in the HELO / EHLO command
to the mail server at mail.example.com, use:

curl smtp://mail.example.com/client.example.com

No MX lookup!

When you send email with an ordinary mail client, it will first check for an
MX record for the particular domain you want to send email to. If you send an
email to joe@example.com, the client will get the MX records for example.com
to learn which mail server(s) to use when sending email to example.com users.

curl does no MX lookups by itself. If you want to figure out which server to
send an email to for a particular domain, we recommend you figure that out
first and then call curl to use those servers. Useful command line tools to
get MX records with include ‘dig’ and ‘nslookup’.