Switching to GNOME Evolution
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Up until recently, Mozilla Thunderbird had been my mail client.
The reason I decided to switch is that I cannot find the way to make
the default delivery format of an outgoing message "Auto-Detect".
When I send an e-mail to a mailing list, the preferred format is
plain-text. But, when I set the domain of a mailing list to be a
plain-text domain, I find that the message is not sent unless I choose
"Auto Detect" or "Plain Text" as the delivery format, but not by default.
It is not convenient at all!
If there's an add on that makes auto-detect the default format, I cannot
find it.
In Evolution, OTOH, plain-text is the default format.
Evolution does not have all the capabilities Thunderbird has. For example,
the direction of a written message is always left-to-right, but Meh! I
can use an external editor if I want to compose a bidirectional message.
But, you have to distinguish between a preference and a plugin: to 
define your external editor, you have to choose 

  Edit->Plugins->External Editor

and not Edit->Preferences.

You can find that option by clicking F1 or Help->contents.
That's always a good idea when you install software.