Add validation for custom SMTP auth type in client tests

Previously, only the presence of the SMTP auth method was checked, but not its type. This additional validation ensures that the SMTP auth type is correctly set to custom, thereby improving test accuracy.

client_test.go

@@ -602,6 +602,10 @@ func TestSetSMTPAuthCustom(t *testing.T) {
 			if c.smtpAuth == nil {
 				t.Errorf("failed to set custom SMTP auth method. SMTP Auth method is empty")
 			}
+			if c.smtpAuthType != SMTPAuthCustom {
+				t.Errorf("failed to set custom SMTP auth method. SMTP Auth type is not custom: %s",
+					c.smtpAuthType)
+			}
 			p, _, err := c.smtpAuth.Start(&si)
 			if err != nil {
 				t.Errorf("SMTP Auth Start() method returned error: %s", err)