There are 3 options for this case;
1- You may have exceeded your daily sending limit in Gmail, Outlook or custom providers.2- If you have created a new email account in Gmail or Outlook, you need to verify your email address before sending.3- If you try to send hundreds of emails at once with a new email account, your email account may have been blocked. Gmail or Outlook treats this process as a suspicious activity. Please check your inbox.
Check all the situations and if there is something different from these situations, please contact us.