Message insert - adding external label - gmail-api

Question

I'm using gmail API to insert an email to the inbox. When the From domain is outside the organization, I expect the external label to be added but it's not.
I'm using python and everything else is working, I am using a service account and able to impersonate on behalf the user's email and I'm using labelIds of ['INBOX', 'UNREAD'] and I need the newly external label as well but couldn't figure a way to add it through the API.
This feature is turned ON for the workspace.
Update - adding my code:
from googleapiclient import discovery, errors
from google.oauth2 import service_account
from email.mime.text import MIMEText
import base64
SERVICE_ACCOUNT_FILE = 'insert-messages-91e77b62878f.json'
SCOPES = ['https://www.googleapis.com/auth/gmail.insert']
def validationService():
# Set the credentials
credentials = service_account.Credentials.\
from_service_account_file(SERVICE_ACCOUNT_FILE, scopes= SCOPES)
# Delegate the credentials to the user you want to impersonate
delegated_credentials = credentials.with_subject('<some_user>')
service = discovery.build('gmail', 'v1', credentials=delegated_credentials)
return service
def SendMessage(service, message):
message = service.users().messages().insert(userId='me', body=message).execute() # me will use <some_user> from above
return message
def CreateMessage(sender, to, subject, message_text):
message = MIMEText(message_text)
message['To'] = to
message['From'] = sender
message['Subject'] = subject
return {'raw': base64.urlsafe_b64encode(message.as_string().encode()).decode(), 'labelIds': ['INBOX', 'UNREAD']}
def main():
try:
service = validationService()
email = CreateMessage('some#external.com', "<some_user>", "Test", "This is a test")
email_sent = SendMessage(service, email)
print('Message Id:', email_sent['id'])
except errors.HttpError as err:
print('\n---------------You have the following error-------------')
print(err)
print('---------------You have the following error-------------\n')
if __name__ == '__main__':
main()