Which key is used when more than one are valid?

[Jean-David Beyer]

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My gnupg file that I get with edit-keys myuid
contains, among other things:

sub  2048g/48FF0850  created: 2007-02-24 expires: 2008-02-24
sub  4096g/124E0663  created: 2007-06-17 expires: 2009-06-16

How do I know which key is used when sending e-mail?
Or is this a Thunderbird question?

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  .~.  Jean-David Beyer          Registered Linux User 85642.
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