STARTTLS extension not supported by server while attaching more than 1 mb file

I am getting "STARTTLS extension not supported by server" while attaching more than 1 MB file with the mail. While attaching less than 1 MB file the exception is not being generated in python.

if file_size <= ATTACHMENT_FILE_SIZE :
            print('in if condition')
            logger.info(f'{thread_name} : In If Condition')
            mailDetails.attach(MIMEText(body_to_be_sent, "html"))
            attachment = open(filepath, 'rb')
            payload = MIMEBase('application', 'octet-stream')
            payload.set_payload(attachment.read())
            encoders.encode_base64(payload)
            payload.add_header('Content-Disposition', "attachment; filename= %s" % f'''{file_name}.xlsx''')
            mailDetails.attach(payload)
            try:
                connection = smtplib.SMTP(SMTPserver)
                connection.ehlo()
                connection.starttls()
                connection.set_debuglevel(False)
                connection.login(SMTPUSERNAME, SMTPPASSWORD)

                text = mailDetails.as_string()
                connection.sendmail(FROM_MAIL_ADDRESS,toAddress.split(",") + ccAddress.split(",")+bccAddress.split(","), text)
                logger.info(f'{thread_name} : Mail Sent successfully')
                # os.remove(filepath)