RacerMail, hosted by Google Apps for Education, has limits that may affect users of the web interface, as well as other clients such as Outlook. The following Google pages will explain more:
Sending limits (Google: To keep our systems healthy and your accounts safe, Google Apps limits the amount of mail one user can send. Limits restrict the number of messages sent per day and the number of recipients per message. After reaching one of these limits, a user can't send new messages for up to 24 hours, but they can still access their account and receive incoming email.)
Bandwidth limits (Google: To help keep our systems healthy and your account safe, all Google Apps accounts have a limited amount of bandwidth. Certain activities that transfer large amounts of data in a short period of time, like synchronizing a Gmail account to a mobile phone or mail client, can cause an account to reach the bandwidth limit.)
Receiving limits (Google: To keep our systems healthy and your accounts safe, all Google Apps accounts limit the amount of email a user can receive per minute, hour, and day. Setting up the account to receive frequent automated messages, such as log files, increases the likelihood of reaching the limits.)
Email attachment size limits and troubleshooting
Why some file types are blocked in emails
More help on email attachments in Google