I found the solution: https://ubuntuforums.org/showthread.php?t=2373908&p=13696490#post13696490
-- You received this bug notification because you are a member of Desktop Packages, which is subscribed to thunderbird in Ubuntu. https://bugs.launchpad.net/bugs/1722518 Title: Tunderbird duplicate email sent Status in thunderbird package in Ubuntu: New Bug description: I use Thunderbird to check my e-mail using IMAP. When I send an e-mail to someone it sends two emails and when I check in the web client there are indeed two sent items. I don't use any email filters just Enigmail for encryption. ProblemType: Bug DistroRelease: Ubuntu 16.04 Package: thunderbird 1:52.3.0+build1-0ubuntu0.16.04.1 ProcVersionSignature: Ubuntu 4.10.0-35.39~16.04.1-generic 4.10.17 Uname: Linux 4.10.0-35-generic x86_64 AddonCompatCheckDisabled: False ApportVersion: 2.20.1-0ubuntu2.10 Architecture: amd64 AudioDevicesInUse: USER PID ACCESS COMMAND /dev/snd/controlC0: ccheco 3091 F.... pulseaudio BuildID: 20170913235937 Channel: Unavailable CurrentDesktop: MATE Date: Tue Oct 10 07:58:37 2017 EcryptfsInUse: Yes Extensions: extensions.sqlite corrupt or missing ForcedLayersAccel: False IfupdownConfig: # interfaces(5) file used by ifup(8) and ifdown(8) auto lo iface lo inet loopback IncompatibleExtensions: Unavailable (corrupt or non-existant compatibility.ini or extensions.sqlite) InstallationDate: Installed on 2017-09-18 (22 days ago) InstallationMedia: Ubuntu-MATE 16.04.3 LTS "Xenial Xerus" - Release amd64 (20170801) IpRoute: default via 192.168.1.1 dev enp1s0 proto static metric 100 default via 192.168.1.1 dev wlp2s0 proto static metric 600 169.254.0.0/16 dev enp1s0 scope link metric 1000 192.168.1.0/24 dev enp1s0 proto kernel scope link src 192.168.1.21 metric 100 192.168.1.0/24 dev wlp2s0 proto kernel scope link src 192.168.1.40 metric 600 Locales: extensions.sqlite corrupt or missing Plugins: Shockwave Flash - /usr/lib/flashplugin-installer/libflashplayer.so PrefSources: prefs.js /usr/lib/thunderbird/extensions/{e2fda1a4-762b-4020-b5ad-a41df1933103}/defaults/preferences/lightning.js /usr/lib/thunderbird/extensions/{a62ef8ec-5fdc-40c2-873c-223b8a6925cc}/defaults/preferences/preferences.js /usr/share/xul-ext/enigmail/defaults/preferences/enigmail.js /usr/share/xul-ext/enigmail/defaults/preferences/000system.js Profiles: Profile0 (Default) - LastVersion=52.3.0/20170913235937 (In use) RunningIncompatibleAddons: False SourcePackage: thunderbird Themes: extensions.sqlite corrupt or missing UpgradeStatus: No upgrade log present (probably fresh install) dmi.bios.date: 03/16/2011 dmi.bios.vendor: INSYDE dmi.bios.version: V1.25 dmi.board.asset.tag: Base Board Asset Tag dmi.board.name: JM51_CP dmi.board.vendor: Acer dmi.board.version: Base Board Version dmi.chassis.type: 10 dmi.chassis.vendor: Chassis Manufacturer dmi.chassis.version: Chassis Version dmi.modalias: dmi:bvnINSYDE:bvrV1.25:bd03/16/2011:svnAcer:pnAspire5820T:pvrV1.25:rvnAcer:rnJM51_CP:rvrBaseBoardVersion:cvnChassisManufacturer:ct10:cvrChassisVersion: dmi.product.name: Aspire 5820T dmi.product.version: V1.25 dmi.sys.vendor: Acer To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/thunderbird/+bug/1722518/+subscriptions -- Mailing list: https://launchpad.net/~desktop-packages Post to : desktop-packages@lists.launchpad.net Unsubscribe : https://launchpad.net/~desktop-packages More help : https://help.launchpad.net/ListHelp