How to check if API key of Elastic Email API is correct or incorrect in asp.net and C# -

i using elastic email api in web application project in asp.net c#. when give username , api_key other required data, email sent destination (i sending email myself testing). , on receiving mail, there 1 lable on page "mail sent successfully"

now problem is, if provide wrong username , api_key, lable not mail. know because have provided wrong api_key.

so how check if have provided valid username , api_key can change lable "check data"

my code:

elasticemail.cs

public static string saveandtestelasticemail(string from, string to, string fromname, string subject, string bodyhtml)         {             try             {                 setelasticemailsettings();                 string channel = system.configuration.configurationmanager.appsettings["registrationnumber"];                 webclient client = new webclient();                 namevaluecollection values = new namevaluecollection();                 values.add("username", username);                 values.add("api_key", api_key);                 values.add("from", from);                 values.add("from_name", fromname);                 values.add("subject", subject);                 values.add("body_html", bodyhtml);                 values.add("channel", channel);                 values.add("to", to);                 byte[] response = client.uploadvalues("https://api.elasticemail.com/mailer/send", values);                 return encoding.utf8.getstring(response);             }             catch (exception ex)             {                 return null;              }          } 

if provide valid api_keyand debug code, getting response{[36]} . , if provide wrong key, still receive response is, response{[22]} , not mail.

    byte[] response = client.uploadvalues("https://api.elasticemail.com/mailer/send", values);                          if (response.length <= 35)                         {                             return null;                         }                         else                         {                             return encoding.utf8.getstring(response);                         }