"You;ve reached the limit" error message

Hi there,

My co-worker is unable to send out any emails from her account.  When she attempts to, she receives the following error message:

"This message can't be sent.  You've reached the limit on the number of messages that can be sent in a 24 hour period"

Is there a quicker way to resolve this issue?  Your prompt assistance on this matter is much appreciated.

Thank you,

Moe