Currently Road Runner Mail Service has a "maximum quota" of 10 MB.
Also, RR has a maximum email file size limit, for both incoming and outgoing mail, of 5MB.
If you receive an email stating:
Then use RR webmail to make sure files have been cleared (not all but enough to send your current attachment/message) off the mail server.
Warning: Your e-mail storage quota has been exceeded. This quota includes all folders, not just your Inbox. Please save your e-mail to your PC and remove it from the server or new mail may not be received.
Note: To access webmail, please refer here: /faq/8902