Save matching values into a variable - arrays

Question

I'm trying to script a "password expiry notification". I collect all users in our AD and put the date, where the password expires into an array. Now my idea is to check if todays date + 4 days is matching with one or more values from the array. It returns "Exists"
Now my question is: How can I save the matching dates/values into a variable?
$Users =#()
$Users += Get-ADUser #QueryParameters | Select-Object -Property $SelectionProperties
for($i = 0; $i -lt $Users.Count; $i ++)
{
$a = $Users[$i].PasswordExpiry
$a
}
$today = (get-date).AddDays(4).ToString('dd/MM/yyyy')
if ($Users.PasswordExpiry -contains $today)
{
write-host "Exists"
}

Answer 1

Use Where-Object to filter out all the users except for those who's passwords expire within 4 days, then send an email to each of those:
$now = Get-Date
$threshold = $now.AddDays(4).Date
$expiringUsers = Get-ADUser #QueryParameters | Where-Object { $_.PasswordExpiry -gt $now -and $_.PasswordExpiry -le $threshold }
foreach($user in $expiringUsers){
$expiryDate = $_.PasswordExpiry
Send-MailMessage -From noreply#domain.tld -To $user.Mail -SmtpServer smtphost.domain.tld -Subject "Password about to expire!" -Body "Your password will expire on '$expiryDate'! Change it today!"
}