special thanks to user "jaykcod"!
Example1: Firefox,Only
To set Outlook as the default mail client for opening up "mailto" links in Firefox (version 15 as of writing this example), first create the following simple bash script in a text editor:
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#!/bin/bash
# Grabs the mailto link and parses it to Outlook 2010 in CrossOver
/opt/cxoffice/bin/wine --bottle "Microsoft Office 2010" --cx-app OUTLOOK.EXE -c IPM.Note /m "$1"
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Note that the above script assumes that Crossover was installed via the .rpm or .deb package. If you installed via the .bin script, then "/opt/cxoffice/bin/wine" would read "~/cxoffice/bin/wine".
Save this file as "mailto.sh", and set the permissions on the file to executable. It doesn't really matter where you save the file to, as long as it's in your home directory.
Next, in Firefox's Edit>Preferences menu, go to the Applications tab and in the "mailto" section, choose "use other" and then browse to and select the "mailto.sh" file. (Note that if the file isn't set to executable, Firefox won't be able to select it).
That's it! If Outlook is already launched and running, "mailto" links will open new messages set to mail to the address in question. If Outlook is not already running, a message will launch and you can compose and "send", but it will remain in your outbox until the next time you fully open Outlook.
Example 2: Chrome/Chromium, etc.
To set Outlook as your default "mailto" client for Chrome/Chromium, etc (as well as Firefox) the process is a bit more involved. To start, create the "mailto.sh" file as instructed in Example 1, above, and set the permissions to executable. Then:
1) Navigate to /home/<user>/.local/shares/applications
2) Find Outlook*2010*.desktop file (i.e.: cxmenu-cxoffice-2481db0b-86ff-4c40-8158-be1d59e42056-1qv24ea-Microsoft+ Outlook+2010.desktop)
3) Copy the file and rename by replacing .desktop with mailto.desktop (i.e. the new file will be something like cxmenu-cxoffice-2481db0b-86ff-4c40-8158-be1d59e42056-1qv24ea-Microsoft+ Outlook+2010+mailto.desktop)
4) Edit this new file in a text editor, and change the reading for the "Exec=" field to the path of the "mailto.sh" script with an added space and "%u", for example
Exec=/home/jack/scripts/mailto.sh %u
then save the file.
5) (optional) Change the Icon= to the location of an icon you want to use for this mime type
6) Edit mimeapps.list and replace whatever is after x-scheme-handler/mailto= to the new file you created. If the previous line ended in a semi colon then be sure to put the semi colon back in. This will be the case with the "mailto=" under "[Added Associations]"