We believe that your issue has been closed by the upload of
Version 0.66 of grml-debootstrap from Michael Prokop <mika@grml.org>.
The explanation is attached below

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Hash: SHA1

Format: 1.8
Date: Thu, 02 Oct 2014 11:39:54 +0200
Source: grml-debootstrap
Binary: grml-debootstrap
Architecture: source all
Version: 0.66
Distribution: unstable
Urgency: medium
Maintainer: Grml Team <team@grml.org>
Changed-By: Michael Prokop <mika@grml.org>
Description:
 grml-debootstrap - wrapper around debootstrap for installing pure Debian
Changes:
 grml-debootstrap (0.66) unstable; urgency=medium
 .
   [ Michael Prokop ]
   * [cd8a1e5] Provide 'make shellcheck' for syntax checks, enable during
     package build
   * [6efce15] Execute wrap-and-sort on debian directory
   * [856786f] Bump Standards-Version to 3.9.6
 .
   [ Patrick Schleizer ]
   * [430c060] Adjust locale handling (LANG/LANGUAGE) to match with Debian
     wheezy defaults
 .
   [ Markus Rekkenbeil ]
   * [9b9cdfe] Add set process for /etc/mailname [Closes: issue1315]
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This is fixed in Git, thanks to bionix for the fix!

Thanks for the hint. I fixed the mixup @ Github.

Looks good to me (except for the variable name mixup; details in the pull request's 
comments section).

I found a working solution to support setting the /etc/mailname,if you use the option -
-hostname in the grml-debootstrap command.
https://github.com/grml/grml-debootstrap/pull/34
Please checked it. My tests are working.

Yes, the /etc/mailname is generated by postfix postinstall script.
The chroot-script from grml-debootstrap set the /etc/hosts and /etc/postfix/main.cf via 
search-and-replace (buzzword: grml).

Maybe a fix is something like this in the chroot-script file (at hostname function)...
echo `hostname -f` > /etc/mailname

As far as I can tell, /etc/mailname is created by the postfix postinstall script.

root@foobar ~ # debconf-get-selections | grep mailname
postfix	postfix/mailname	string	grml.mycurrentdomain.tld

I'm not sure whether the mail name should be fixed by preseeding or if it should be set 
in the hostname function in chroot-script, too.

Hi,
I checked the chroot-script and the current function for setting the hostname set no 
/etc/mailname.

The device paths in grml-debootstrap should be /dev/vda and /dev/vda1, of course. Sorry 
about that.

When using grml-debootstrap to deploy wheezy on a grml 2014.03 system, the mail name in 
/etc/mailname is always set to "grml.mycurrentdomain.tld", no matter what hostname is 
supplied to grml-debootstrap via the "--host" command line option.

Steps to reproduce on a grml 2014.03 system:

root@grml ~ # echo ';' | sfdisk /dev/vda
root@grml ~ # grml-debootstrap --arch amd64 --filesystem ext4 \
    --grub /dev/sda --hostname foobar --release wheezy \
    --password secret --target /dev/sda1
# [...]
root@grml ~ # hostname
grml
root@grml ~ # mount /dev/vda1 /mnt
root@grml ~ # cat /mnt/etc/mailname
grml.mycurrentdomain.tld

What I would expect to see:

root@grml ~ # cat /mnt/etc/mailname
foobar.mycurrentdomain.tld