I'm wondering how it's possible to remove the little mail indicator applet in the global menu. I've uninstalled all the programs that use it, like Thunderbird, Empathy, and Gwibber, since I don't use any of them, but the icon is still there.
Any ideas?
You can remove the message indicator by removing the indicator-messages 
package by clicking on that link and clicking uninstall. Or by executing the following command in a terminal:
sudo apt-get remove indicator-messages
Then log out and back in.
If Thunderbird Mail is installed and is the default mail application in system settings->Details: then start Thunderbird, go to menu->Tools->Add-ons->Extensions and disable Thunderbird extension "Messaging Menu and Unity Launcher integration". Quit Thunderbird. Logout and Login to Ubuntu.
This solved it on Ubuntu 14.04 and 16.04 LTS without the need to uninstall anything.
Remove the package indicator-messages from within the Ubuntu Software Center.
Then reboot, gone!
This instruction will not remove the sound icon on Ubuntu 12.04 (Precise)
The program can be uninstalled from your system by opening the program "terminal", and typing in
sudo apt-get remove indicator-messages -y
It will ask you for your password, and then it will remove the program from your system.
The messages indicator only appears when an application has registered to use it; by default, no applications are registered to use it, meaning that the messages indicator is hidden. However, some applications, such as Thunderbird, register with the indicator as soon as you open them and leave no apparent way to remove them.
My solution is to reset the list of applications registered with the indicator to the default (empty), then kill the indicator to refresh it.
Open a terminal and copy/paste the following:
dconf reset /com/canonical/indicator/messages/applications
killall indicator-messages-service
On 12.04 (precise):
sudo apt-get remove -y indicator-messages
killall indicator-messages-service
killall unity-panel-service
There is a word of caution for removing the e-mail icon. You will also loose the Sound Control icon as both are connected to each-other. By removing the "indicator applet" package you will no longer be able to access or control the sound settings by clicking on the panel icon.
this works in the gnome shell for 11.04, not sure about Unity interface:
open synaptic + remove "indicator-messages" + log out and log back in
1) In a terminal:
sudo apt-get remove indicator-messages
sudo killall gnome-panel
This will remove the package, and then remove the icon without rebooting.
So doing a removal of "indicator-messages" will result in removing more then you want like the sound indicator and others. for a list of what you might remove start dconf editor and look under /com/canonical/indicator/messages/applications to see what is there. Having made this mistake of removing indicator-messages" the best solution is to edit /com/canonical/indicator/messages/applications and only remove what you want. In my situation I only had to reset (Default:[])/com/canonical/indicator/messages/applications/thunderbird.destop that set this to blank. re:List of applications that are shown in the messaging menu, Applications corresponding to the desktop file ids in this list are shown in the messaging menu. SO I vote for "Hitechcomputergeek contribution number 5
Run sudo apt-get remove indicator-messages and then either log out or run killall unity-panel-service for the changes to take effect.
On Ubuntu 10.10 or earlier versions click with right mouse button on the icon and click on Remove From Panel
On Ubuntu 11.04 (Unity) click with right mouse button on the icon and click on Keep In Launcher Removing icons on Ubuntu 11.04 Classic works the same as on Ubuntu 10.10
