When using multiple rules and different UseAdditionalField per rule module will fail

[PrzemyslawKlys]

[ordered] @{
    Enable               = $false # doesn't processes this section at all if $false
    RemindersDisplayOnly = $false # prevents sending any emails (good for testing) - including managers
    SendToDefaultEmail   = $true # if enabled $EmailParameters are used (good for testing)
    Reminders            = 1, 7, 14, 15 # both dictionary and array works for reminders
    UseAdditionalField   = 'extensionAttribute13'
    SendCountMaximum     = 3
}
[ordered]@{
    Enable               = $false # doesn't processes this section at all if $false
    RemindersDisplayOnly = $false # prevents sending any emails (good for testing) - including managers
    SendToDefaultEmail   = $true # if enabled $EmailParameters are used (good for testing)
    Reminders            = 3, 9
    UseAdditionalField   = 'extensionAttribute12'
    SendCountMaximum     = 3
}

The code checks AdditionalProperties for User & Manager, but it assumes it's a string - not multiple strings

PSPasswordExpiryNotifications/Public/Find-PasswordExpiryCheck.ps1

Lines 68 to 85 in 66cf5b7

	$UserManager = $CachedUsers["$($_.Manager)"]
	if ($AdditionalProperties) {
	# fix this for a user
	$EmailTemp = $_.$AdditionalProperties
	if ($EmailTemp -like '*@*') {
	$EmailAddress = $EmailTemp
	} else {
	$EmailAddress = $_.EmailAddress
	}
	# Fix this for manager as well
	if ($UserManager) {
	if ($UserManager.$AdditionalProperties -like '*@*') {
	$UserManager.Mail = $UserManager.$AdditionalProperties
	}
	}
	} else {
	$EmailAddress = $_.EmailAddress
	}

This will require some advanced logic depending on which rule we're working on. Not a top priority as I guess it would be not likely that multiple rules would be running within the same domain with different overwrite email fields.