/usr/share/xfce4/helpers/mutt.desktop is in libexo-helpers 0.10.2-3ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 | [Desktop Entry]
Version=1.0
Icon=mutt
Type=X-XFCE-Helper
Name=Mutt
Name[ast]=Mutt
Name[eu]=Mutt
Name[pt_BR]=Mutt
StartupNotify=false
X-XFCE-Binaries=mutt;
X-XFCE-Category=MailReader
X-XFCE-Commands=exo-open --launch TerminalEmulator %B;
X-XFCE-CommandsWithParameter=exo-open --launch TerminalEmulator /usr/lib/x86_64-linux-gnu/xfce4/exo-1/exo-compose-mail-1 mutt %B "mailto:%s";
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