Friday, October 21, 2016

Oracle Linux : sending mail with Sendmail

There can be many reasons why you need to send mail from your Linux host to some mail account. For Example, you have an application that needs to send out mail to end users, in those cases you will use a central SMTP mail relay server within your corporate IT footprint. However, in some cases you want to have scripting that makes use of a local SMTP instance that will send the mail for you. This can be direct to the end user or using a SMTP relay server.

In cases you want to have your local Linux machine to send out the messages directly to the recipient you will have to ensure that (A) your machine is allowed to make the connection outside of your firewall to the recipient mail server and (B) you will have to make sure you have a local MTA (Mail Transfer Agent) in place. The best known MTA’s are Sendmail and Postfix. We will use Sendmail as an example while showing how to send mails from an Oracle Linux machine to a gmail account (or whatever account you require) by using simple bash commands and scripting.

Install Sendmail on Oracle Linux
Installing Sendmail is most likely the most easy step in the entire blogpost. You can install Sendmail by making use of the default Oracle Linux YUM repositories. Install Sendmail is done with the below command. You will notice we install Sendmail and sendmail-cf. Sendmail-cf is used to make your life much more easy when configuring and reconfiguring Sendmail.

yum install sendmail sendmail-cf

For some reason Sendmail migt be giving you some strange errors every now and then right after you installed it and start using it. A good practice to ensure everything is ready to go is to stop and start the sendmail service again as shown in the example below.

[root@testbox08 log]#
[root@testbox08 log]# service sendmail status
sendmail (pid  968) is running...
sm-client (pid  977) is running...
[root@testbox08 log]#
[root@testbox08 log]# service sendmail stop
Shutting down sm-client:                                   [  OK  ]
Shutting down sendmail:                                    [  OK  ]
[root@testbox08 log]#
[root@testbox08 log]# service sendmail start
Starting sendmail:                                         [  OK  ]
Starting sm-client:                                        [  OK  ]
[root@testbox08 log]# service sendmail status
sendmail (pid  1139) is running...
sm-client (pid  1148) is running...
[root@testbox08 log]#
[root@testbox08 log]#

After this your sendmail installation on Oracle Linux should be ready to go and you should be able to send out mails. We can easy test this by sending a test message.

Sending your first mail with sendmail
Sending mail with Sendmail is relative easy and you can make it even easier by ensuring your entire message is within a single file. As an example, I created the file /tmp/mailtest.txt with the following content:

Subject: this is a test mail

this is the content of the test mail

This would mean the mail is send to my gmail account, the subject should be “this is a test mail” and the body of the mail will show ” this is the content of the test mail”. Sending this specific mail (file) can be done by executing the below command:

[root@testbox08 tmp]# sendmail -t /tmp/mailtest.txt

However, a quicker way of ensuring your message is processed is removing the “To:” part and using a command like shown below:

[root@testbox08 log]# sendmail < /tmp/mailtest.txt

The below screenshot shows that the mail has arrived in the mailbox, as expected. You can also see it has gotten the name of the account and the fully qualified hostname from the box I used to send the mail from. In this case this shows a Linux host located in the Oracle Public cloud.

Making your reply address look better
The above mail looks a bit crude and unformulated. Not the mail you would expect to receive as an end user, and especially not as a customer for example. Meaning, we have to make sure the mail that is received by the recipient is formatted and in a better way.

The first thing we like to repair is the name of the sending party. We would, as an example, have the name shown as "customer service" and the reply address should become To do so we add a "Reply" line to the /tmp/mailtest.txt file which looks like:

From: customer service

Due to the formating it is not showing as it is rather showing in the way we commonly see and as is shown in the screenshot below:

Giving the mail priority
Now, as this is a mail from customer service informing your customer that his flight has been canceld  it might be appropriate to this mail a priority flag.

Doing more with headers
In essence you can define every mail header you like and which is understandable and which is allowed. To get an understanding of the type of headers that you can use and which are common you can have a look at RFC 2076 "Common Internet Message Headers".

Sending HTML formatted mail
It is quite common to use HTML to format emails. Ensuring you can send your email in a HTML formatted manner requires that you have the right headers in your email and you format your message in the appropriate HTML code (please review the example on github).

An important thing to remember is that not everyone is able to read HTML. For this it is good to use the "content-Type: multipart/alternative;" header in combination with the "Content-Type: text/html; charset=UTF-8". This will allow you to make a distinct between HTML formatted mail and non-HTML formatted mail.

All the examples below can be found in the example mail file "/tmp/mailtest.txt" which is available on github.
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