[TYPO3] Tmailform - Problems using checkbox-values

[Bernhard]

Hi!

I'm using the extension TMailform for my mail-forms, sending the mails 
in plain-text. Everything seems to work fine, just one single checkbox 
is not working as it should.
This is the (imho) relevant part of my template:

<!-- ###TMAIL_FORM### begin -->
...
<input type="checkbox"
name="tx_pilmailform_pi1[checkbox][einverstanden]" value="yes"
###EINVERSTANDEN_YES_VAL### />
...
<!-- ###TMAIL_FORM### end -->

<!-- ###TMAIL_MAIL### begin -->
...
# Einverstaendnis:      ###EINVERSTANDEN_YES_VAL###
...
<!-- ###TMAIL_MAIL### end -->

If the form ist submitted, the value "yes" - which should be in the Mail 
instead of the marker ###EINVERSTANDEN_YES_VAL### if the checkbox is 
checked - is missing.

I would be very happy to be given some advice, what I am doing wrong.

Thanks in advance,
Bernhard