Hide the user agent when sending emails with Thunderbird

Martin Brinkmann
Sep 8, 2012
Email, Thunderbird
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9

Thunderbird, and other mail clients as well, include information in each email you sent that reveal information about your operating system, the mail program and the version of it that you are using. While that is certainly not an issue if you are sending an email to your mum, close friends or other contacts you know, you are better of not to reveal what you are using to someone that you do not trust or know.

The reason for this is simple: The operating system and mail client can provide malicious users with information that they can use to attack your system. Say they notice that you are running an older version of Thunderbird or another email client with known vulnerabilities. They can then use the information to attack your system efficiently thanks to those information that you have provided them with.

thunderbird user agent

You can check the user agent in Thunderbird by either enabling full headers under View > Headers > All, or by clicking on Other Actions > View Source.

The user agent on the screenshot above reveals the operating system, Windows 7 64-bit, the email client and the version of that client.

There is unfortunately no switch in the program's settings to change the user agent in Thunderbird. What you need to do is add a hidden parameter to the advanced configuration to do that. Here is how this is done.

Open the settings of the email program with a click on Tools > Options. Switch to the Advanced tab here and click on Config Editor under General. You may see a warning message if that is your first time opening the about:config window. If that is the case click on I'll be careful to continue.

Right-click in the parameter window and select New > String from the options. Enter general.useragent.override in the first form that pops up, and nothing in the second. This sets the user agent to a blank and that is what recipients see when they analyze the email header.

thunderbird block user agent

It is unfortunately not that easy to remove the preference again. For that you need to edit the prefs.js file in your Thunderbird profile. You should find it under Windows 7 in the C:\Users\username\AppData\Roaming\Thunderbird\Profiles\random.default folder. Locate the general.useragent.override line and delete it completely. Make sure Thunderbird is closed when you edit the file.

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Comments

  1. JMGG said on January 19, 2012 at 8:25 am
    Reply

    You said that Outlook isn’t your main email client, so which is your main one?

    1. BalaC said on January 19, 2012 at 9:42 am
      Reply

      I think its thunderbird

    2. Martin Brinkmann said on January 19, 2012 at 10:15 am
      Reply

      It is Mozilla Thunderbird.

  2. Salaam said on September 24, 2012 at 9:52 pm
    Reply

    Awesome! This actually solved my problem… what a stupid bug.

  3. Claud said on December 19, 2012 at 2:08 am
    Reply

    If this is the same bug that I’ve encountered, there may be another fix: (1) hover over open Outlook item in Taskbar, cursor up to hover over Outlook window item, and right-click; (2) this should give you Restore / Move / Size / Minimize / Maximize — choose Move or Size; (3) use your cursor keys, going arbitrarily N/S/E/W, to try to move or size the Outlook window back into view. Basically, the app behaves as though it were open in a 0x0 window, or at a location that’s offscreen, and this will frequently work to resize and/or move the window. Don’t forget to close while resized/moved, so that Outlook remembers the size/position for next time.

    1. Lynda said on February 12, 2013 at 3:37 pm
      Reply

      THANK YOU Claude!!! I could get the main window to launch but could not get any other message window to show on the desktop. You are my hero!!!!

    2. Chad said on November 20, 2018 at 4:24 pm
      Reply

      Solved my issue! 6 years later and this is still problem…

    3. Ivan X said on January 21, 2021 at 4:50 pm
      Reply

      Fantastic. Thank you. Size did the trick.

  4. Andrew said on October 26, 2013 at 7:06 am
    Reply

    This solved my Outlook problem, too. Thank you. :)

  5. Charles said on December 7, 2013 at 7:23 pm
    Reply

    Thank you so much, this started happening to me today and was causing big problems. You are a life saver, I hope I can help you in some way some day.

  6. garth said on November 7, 2014 at 7:13 pm
    Reply

    You are a god – thank you!

  7. Faisal said on February 9, 2015 at 10:09 am
    Reply

    thanks a lot…. work like charm.. :-)

  8. Simon said on March 24, 2015 at 11:36 pm
    Reply

    Yah…thanks Claude. I’ve been having the same problem and tried all the suggestions…your solution was the answer. It had resized itself to a 0/0 box. Cheers

  9. Olu said on April 14, 2015 at 1:35 pm
    Reply

    Excellent post. This had me baffled even trying to accurately describe the problem. This fixed it for me.
    Thank you

  10. Coenig said on July 23, 2015 at 7:36 am
    Reply

    Thanks a lot for the article. Don’t know why it happenend, don’t know how it got fixed, but it was really annoying and now it works :-)

  11. Fali said on January 20, 2016 at 4:19 pm
    Reply

    Thanks a lot. I was facing this issue from past 3 week. I tried everything but no resolution. The issue was happening intermittently and mainly when I was changing the display of screen ( as i use 2 monitors). The only option i had was to do system restore. But thanks to you.

    1. MIki said on January 10, 2019 at 11:54 am
      Reply

      I’ve been tried to sole this problem for 12hours. Your comment about changing the display of screen helped me a lot!! Thanks!!

  12. Christina said on January 20, 2016 at 6:14 pm
    Reply

    Thank you…don’t know why this happened but your instructions helped me fix it. Running Windows 10 and office pro 2007

  13. Oz said on July 22, 2016 at 3:20 pm
    Reply

    Great tip! Thanks!

  14. Tracy said on September 1, 2016 at 4:48 pm
    Reply

    Worked for me, too – thank you!!!

  15. shawn said on September 9, 2016 at 10:25 am
    Reply

    It’s Worked for me, too
    thank you very much!

  16. Jari said on October 31, 2016 at 11:53 am
    Reply

    I had a similar issue with Outlook 2013 on Windows 10 and this helped me to fix it. Thank you very much!

  17. Michel H said on November 30, 2016 at 11:08 pm
    Reply

    Thank you so much. Solved!
    Considering you published this in 2012, incredible not been debugged by Microsoft.
    Thank you again. M

  18. Ziad Bitar said on January 9, 2017 at 2:00 am
    Reply

    This problem was faced by only one user logging to TS 2008 r2 using outlook 2010.The issue was resolved.

    Thanks.

  19. Anonymous said on February 15, 2017 at 5:24 pm
    Reply

    Great tip. Thank you!!!! If it helps, I had to use the Control Key and the arrow keys at the same time to bring my window back into view. Worked like a charm.

  20. Rochelle said on March 6, 2017 at 11:59 am
    Reply

    Thank you, this worked !!!!

  21. anom1234 said on May 20, 2018 at 11:20 pm
    Reply

    Man, you are a fucking god. Thanks a lot, what an annoying bug!!

  22. JC said on October 12, 2020 at 2:14 pm
    Reply

    Awesome, this post solved the issue. Many thanks!

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