User not receiving quota-related e-mail messages

Try one or more of the following:

Make sure that the correct e-mail address is specified for the user

For more information, see Managing individual users.

Check the user’s enforcement setting

If a global setting or custom quota setting is applied to the user, then set the enforcement setting to Enforce or Inform. For more information, see Configuring account defaults.

Make sure that the e-mail messages are not blocked by a spam filter