Updated RequiredPropertyIsEmptyFormat to be more clear (#242)

Author: snakefoot

src/NLog.MailKit/MailTarget.cs

@@ -85,7 +85,7 @@ public class MailTarget : TargetWithLayoutHeaderAndFooter
         private static readonly Encoding DefaultEncoding = System.Text.Encoding.UTF8;
         private const SecureSocketOptions DefaultSecureSocketOption = SecureSocketOptions.StartTlsWhenAvailable;
 
-        private const string RequiredPropertyIsEmptyFormat = "After the processing of the MailTarget's '{0}' property it appears to be empty. The email message will not be sent.";
+        private const string RequiredPropertyIsEmptyFormat = "MailTarget '{0}'-property appears to be empty. The email message cannot be sent.";
 
         /// <summary>
         /// Initializes a new instance of the <see cref="MailTarget" /> class.