As part of keeping security measures current, Microsoft will be removing support for legacy authorization (logins) for email on phones. If you're using the Outlook app for email on your phone already, this should not apply to you.
Android Users: Android mail apps do not support secure authorization for Microsoft email accounts and therefore you will need to remove the account from your Android mail accounts and install the Outlook app from the Play store.
iPhone Users: You can use either the default mail app or the Outlook app. In either case, you will need to remove your LETU mail account and then re-configure it to let the appropriate authentication take place. (steps below for iPhone)
To ensure your phone is using the most current security for LETU email, you'll need to first remove your existing LETU email account from your phone by proceeding through the following steps and then setup email again either by configuring iPhone email to use the Outlook.com 'Exchange' setting or by installing Outlook for Android or Outlook for iPhone/iOS.