Well you could use preg for this but this could do just fine:
$tmp=explode("\n",$mailbody);
foreach ($tmp as $v) {
$tmp2=explode(': ',$v);
$arr[chop($tmp2[0])]=$tmp2[1];
}
print_r($arr); // just to show what kind of array you will have
Result for that would be:
Array
(
[Date] => 12.06.2003
[Artikelbezeichnung] => Li-ion Akku für Nokia 6210 / 6310 /7110 / 1700mAh Power
[Artikelnummer] => 6394115339
[Anzahl der Gebote] => 1
[Verkaufer] => xxx
[E-Mail-Adresse des Verkäufers] => bla@bla.com
[Kaufer] => blabla
[E-Mail des Kaufers] => xxx@yyy.com
)