if ($email_address eq 'first.last@example.com') {
    # OK - process normally
}
else {
    # Possibly not OK - phishing check
}

##</code><code>##

my %valid_email_for = (
    'Display Name' => {
        'first.last@example.com' => 1,
        'first.last@example.net' => 1,
        'first.last@example.org' => 1,
    },
);
...
if (exists $valid_email_for{$displayname}{$email_address}) {
    # OK - process normally
}
else {
    # Possibly not OK - phishing check
}