Issues with email subject search criteria

[ardudi]

I use ESP8266 mini board, latest ESP_Mail_Client version.
I use the code below to search for a specific subject content. There are two options, either system Mac address ot a predefined userEmail. The app uses Gmail.

When app calls serchSubject function to search for Mac address and no email found in Gmail inbox, on the serial optput I get #### Searching messages...

no messages found for the specified search criteria.

When app calls serchSubject function to search for userEmail, I get: #### Searching messages...

Error, SEARCH not allowed now.

---> directly from IMAP client error code: -203

Why? What is wrong? what is the defference?

char userEmail[50];

void serchSubject(bool specialUser)
{
if (specialUser)
{
IMAP_Setup(false);
}
imap_data.fetch.uid.clear();
imap_data.fetch.number.clear();
imap_data.fetch.sequence_set.string.clear();

if (specialUser)
{
imap_data.search.criteria = F("SEARCH SUBJECT ") + WiFi.macAddress();
}
else
{
imap_data.search.criteria = F("SEARCH SUBJECT ") + String(userEmail);
}

Serial.print(F("---> Search criteria: "));
Serial.println(imap_data.search.criteria.c_str());

if (!MailClient.readMail(&imap, false))
{
String errorMsg = imap.errorReason();
Serial.print(F("---> directly from IMAP client error code: "));
Serial.println(imap.errorCode());
}

for (int i = 0; i < max_result; i++)
{
imap_data.search.criteria.clear();
// Now Fech message by UID stored in msg_uid
imap_data.fetch.uid = msg_uid[i];
MailClient.readMail(&imap, false /* session open for fetching message in opened mailbox later */);
}
imap.closeSession();
imap.empty();
}