Guest accounts without AD listing cannot be searched via the flag `--mail`

An example of how things work at the moment and do not work out as expected for the --mail flag:

$iam persons --mail guest@ethz.ch
[]
$iam persons --username guest
[
    {
        "firstname": "Guenevere",
        "lastname": "Stablished",
        "mail": "guest@ethz.ch",
        "npid": "1",
        "uid": "guest"
    }
]