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Display Mail User Agent T 2.9.13
od Toshi, frightanic
Add-on for Thunderbird that displays information about the mail user agent (i.e. email software) with which the selected mail was composed.
O doplňku
Ever wondered what software someone used to write you an email? Display Mail User Agent T answers that question at a glance. It inspects the headers of every message you open in Thunderbird and shows a small, recognisable icon — and optionally a text label — identifying the email client, webmail service, or marketing platform that sent it.
Why you might want this
Knowing the sender's mail client can be surprisingly useful. It helps you understand why a message looks the way it does (missing formatting, odd line breaks, garbled encoding), spot automated or bulk-mail senders, quickly distinguish personal messages from CRM- or newsletter-generated ones, and satisfy simple curiosity about what tools the people you correspond with actually use.
Features
A note on the project's history
This add-on has a long lineage. It started life in 2005 as "dispMUA" by Christian Weiske, was maintained for many years by Jürgen Ernst, then forked and relaunched in 2019 by Toshi_ as "Display Mail User Agent T" to keep it compatible with modern Thunderbird. The current maintainer took over in March 2026 to ensure the project continues to receive updates. The source code is publicly available on GitHub.
Frequently Asked Questions
I don't see the icon. A character string such as
This happens if the email does not have an
Why you might want this
Knowing the sender's mail client can be surprisingly useful. It helps you understand why a message looks the way it does (missing formatting, odd line breaks, garbled encoding), spot automated or bulk-mail senders, quickly distinguish personal messages from CRM- or newsletter-generated ones, and satisfy simple curiosity about what tools the people you correspond with actually use.
Features
- Identifies the mail user agent from standard email headers (
User-Agent,X-Mailer,X-Mail-Agent,X-MimeOLE,Organization,Message-ID, and more) - Ships with a library of well over 2,000 icons covering email clients, webmail providers, marketing platforms, and other senders — from classic desktop clients like Outlook, Apple Mail, and Evolution to modern services like Gmail, Fastmail, HEY, and Spark
- Displays the icon directly in the message header area so you see it without any extra clicks
- Offers an optional text label alongside the icon for even clearer identification
- Lets you fix the icon position to avoid it overlapping the message body when the message pane is narrow
- Supports user overrides, so you can add or adjust entries for senders not yet in the built-in database
- Comes with a built-in feedback mechanism to report unrecognised agents and help grow the database
- Localised into more than 20 languages
A note on the project's history
This add-on has a long lineage. It started life in 2005 as "dispMUA" by Christian Weiske, was maintained for many years by Jürgen Ernst, then forked and relaunched in 2019 by Toshi_ as "Display Mail User Agent T" to keep it compatible with modern Thunderbird. The current maintainer took over in March 2026 to ensure the project continues to receive updates. The source code is publicly available on GitHub.
Frequently Asked Questions
I don't see the icon. A character string such as
Message-ID or DKIM-Signature is displayed.This happens if the email does not have an
X-Mailer or User-Agent header. Since Display Mail User Agent T identifies the MUA based on these headers, it cannot identify emails that lack them. To investigate, display the raw message source (☰ → View → Message Source or Ctrl+U) and check which headers are present.