If mails from your web-server/webapp is having delivery issues, DKIM (DomainKeys Identified Mail) can help you big time.
Its highly recommended to use DKIM for outgoing emails even if your server is not running any kind of mail-hosting.
apt-get install opendkim opendkim-tools
Edit Config files
Open dkim config file
Add following lines towards end. Make sure you replace example.com with your domain/subdomain.
Domain example.com KeyFile /etc/postfix/dkim.key Selector mail SOCKET inet:8891@localhost
Next open dkim defaults file
Change default socket path by adding a line like below:
Open postfix main config file
Add following lines towards end.
# DKIM milter_default_action = accept milter_protocol = 2 smtpd_milters = inet:localhost:8891 non_smtpd_milters = inet:localhost:8891
DKIM Key Generation
Run following commands with
example.com matching values used in
/etc/opendkim.conf file in earlier step.
opendkim-genkey -t -s mail -d example.com
This command will generate mail.private and mail.txt file. mail.private is private key that will be used to sign outgoing emails. Move it to the location we specified earlier in
cp mail.private /etc/postfix/dkim.key
DNS Record Setup
Next, you need to create a TXT record on DNS end. Just check content of mail.txt file created by opendkim-genkey command we ran above.
You will see something like below:
mail._domainkey IN TXT "v=DKIM1; k=rsa; t=y; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDYv84GSl0Xp2CrPdFqMZ9ShBDi9Pal9XpfIf7asEENxLRdIka3TONpqtrcCKksROJBNh2G3OVGuoGJ1watQGT46B+zQtjcCI67+WiTlb2D98s1UV3KO7oi/0QH/lH8DzUmrGJUIy3ZBQ9mIu1t6YDyi8y3hlhTILHW7G4HV/VtwQIDAQAB" ; ----- DKIM key mail for example.com
TXT record will require NAMS & VALUE.
mail._domainkey for NAME and long string in quotes starting from v=DKIM1 as VALUE.
Below is a sample screenshot for a TXT record. User-interface on your end might differ.
If you are editing a previous DNS record, it might take sometime for changes to propogate.
Once al config & setup done, you need to start DKIM service and restart postfix.
service opendkim start service postfix restart
Testing DKIM setup for correctness
Anything we do, specially for first time, must end with successful testing!
There are many tools for testing. I will mention few of them below.
Verify DNS Records for DKIM Setup
This will ONLY verify if your TXT record is created successfully.
Classic and easy. You must be having this already. Running…
dig mail._domainkey.example.com TXT
should return a response like…
;; ANSWER SECTION: mail._domainkey.exmaple.com. 86400 IN TXT "v=DKIM1\;" "k=rsa\;" "t=y\;" "p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDYv84GSl0Xp2CrPdFqMZ9ShBDi9Pal9XpfIf7asEENxLRdIka3TONpqtrcCKksROJBNh2G3OVGuoGJ1watQGT46B+zQtjcCI67+WiTlb2D98s1UV3KO7oi/0QH/lH8DzUmrGJUIy3ZBQ9mIu1t6YDyi8y3hlhTILHW7G4HV/VtwQIDAQAB"
Web-based Record Check
You can use http://www.protodave.com/tools/dkim-key-checker/
Verify DKIM Signing
Test #1 – Email-based
If you have setup keys correctly then you should pass this test.
You can test by simply sending an email to email@example.com or firstname.lastname@example.org
It’s better to use swaks tools for mail-testing (
apt-get install swaks).
swaks -t email@example.com -f firstname.lastname@example.org
Replace email@example.com with your mail id where you would like to receive test results.
Test #2 – Web-based
Better choice will be to use a service like http://www.mail-tester.com/ which gives you a temporary email ID and web-interface to see what happens to the email on receiving end!
For WordPress, its better to test using Check Email plugin as you will get better picture of what happens to mail sent from WordPress!
in /etc/opendkim.conf to use the same key for multiple domains
and added “no_milters” to /etc/postfix/master.cf
made this line:
This prevented opendkim signing messages twice.
I am curious whether this is sane or not. Remember laziness counts!
Enjoy your day and thank you for your time.
I never used DKIM like above but in another use-case, I used no_milter to avoid double processing.
If it’s working, it must be fine! 😉
Thanks so much. I was stuck with other tutorials at entering the Information into DNS.
Helped me great!!
thank you for this usefull tutorial 😀
i used to create my DKIM
in http://www.protodave.com/tools/dkim-key-checker/ with selector mail is passed ok!
but in the http://www.mail-tester.com/ web site i have always (-1) for this.
i have to use selector named default?
or change something else in settings?
thank you 🙂
You need to make sure selector and DNS TXT name match. If there is a mismatch above testing tool may throw error.
Hi there! I followed your setup to the letter and I am no longer able to send mails. I get a transport error on the Milter serve. Have you got any idea why?
“postfix/cleanup: warning: invalid transport name: “inet in Milter service: “inet:localhost:8891″”
Check if your postfix config matches – https://rtcamp.com/tutorials/mail/dkim-postfix-ubuntu/#postfix-file ?
Ok So I reset all the config and now it’s working. However, I have another blockage: when I do a HELO test via smtp on the server side (i.e. telnet localhost 25) I can send mails. However, I added my distant application site to mynetworks in order to allow smtp relays. The problem is that the HELO test seems to go well and the message is queued, but the server does nothing. The log file gives no error message either… Any idea?
For HELO issue, please check postfix error log.
t=y in DKIM Key enables test mode, so even if you have DKIM, you are telling it to ignore. Also you can see the mails, you’ll get test mode in it. (when you click show original, it displays dkim =pass (test mode) ).
Ref; http://goo.gl/j3Fjbp, http://goo.gl/sbSI82
Very usrful info! Thanks for sharing. 🙂
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