Expected response code "250/251/252" but got code "451", with message "451 Temporary local problem - please try later" - laravel

Question

I am new in Laravel development. I am sending an email to agencies which is approved and email is not null and if agencies email is null then get customer email for send email, But I got this error https://flareapp.io/share/DPygyxQ5#F57
I have tried to figure out where is the issue using echo pre in every steps. I am using smtp for sending email.
If you think that is there issue in smtp so if user register itself then email is successfully sent to user.
Here is code
// Get agency details
$agencies = Agency::where('status', 2)->get();
$property = Property::where('property_id', $id)->first();
if($agencies->isNotEmpty())
{
foreach($agencies as $agency)
{
if($agency->email != null)
{
$agency_name = $agency->name;
$agency_email = $agency->email;
$property_slug = $property->slug;
$property_link = route('property.detail', $property_slug);
Mail::send('emails.user.agency.mail_to_agency_after_property_approve',
[
'agency_email' => $agency_email,
'agency_name' => $agency_name,
'property' => $property,
'property_link' => $property_link,
],
function($message) use ($agency_email)
{
$message->to($agency_email);
$message->subject('Fresh Property Listing Update');
});
}
else
{
$customer = Customer::select('customer_id', 'first_name', 'last_name', 'customer_email')->where('customer_id', $agency->customer_id)->first();
$agency_name = $customer->first_name.' '.$customer->last_name;
$agency_email = $customer->customer_email;
$property_slug = $property->slug;
$property_link = route('property.detail', $property_slug);
Mail::send('emails.user.agency.mail_to_agency_after_property_approve',
[
'agency_email' => $agency_email,
'agency_name' => $agency_name,
'property' => $property,
'property_link' => $property_link,
],
function($message) use ($agency_email)
{
$message->to($agency_email);
$message->subject('Fresh Property Listing Update');
});
}
}
}