To check whats currently whitelisted (Bypassed Recipients):

Get-ContentFilterConfig

To whitelist a single email address:

$list = (Get-ContentFilterConfig).BypassedSenders
$list.add(“new.mail@address.com”)
Set-ContentFilterConfig -BypassedSenders $list

To whitelist an entire domain:

$list = (Get-ContentFilterConfig).BypassedSenderDomains
$list.add(“domain.com”)
Set-ContentFilterConfig -BypassedSenderDomains $list

Source: SBITZ