Outlook also has the option to
set a "default" format for all messages that you create in it's
Tools/Options menu as shown below.
When set as shown above, all
messages will be composed in "Plain Text" unless you select another
format when composing the message as shown in the first Outlook screen
above.
Most EMail Client software has the option to specify what message
format will be used for messages addressed to specific contacts in your
email contact list. Outlook has this, but it's not easy to find.
To set the preferred format for a specific contact in Outlook's contact list;
Open the contact for editing
<doubleclick> the contact's EMail address
Then set the format for that specific contact as shown below
Once this is set, each time
you create a new message or compose a reply, you will be given the
default mail format edit screen. For example, that screen will not have any font or other formatting options.
If, however, you want to "toggle" to the alternate, non default,
message
format, simply hold the <shift> key down when you click "Write"
or "Reply" For example, if you have HTML unchecked as shown
above, holding down the shift key when clicking one of these buttons
will start a compose window with text formatting and other html options.
Thunderbird also has the option to specify what format will be used for
messages addressed to specific contacts in your email contact list.
To set the preferred format for a specific contact in Thunderbird's Address Book, open the contact for editing, then select the format for emails addressed to that contact as shown below.
Note that if you have a
contact set to a specific format, you can still compose messages to
them in the other format by <shift> clicking the write or reply
buttons.