Outlook 2010: Could not complete the operation. One or more parameter values are not valid.

User reports when trying to send a message, they get the error "Could not complete the operation. One or more parameter values are not valid."

She said when she removed some recipients, the message sent, but that could have been a coincidence.

Server is Server 2008 / Exchange 2007.
PC is Windows 7 / Outlook2010

Event viewer on the PC has logged this as event ID 25 multiple times:

Log Name:      Application
Source:        Outlook
Date:          9/20/2010 8:37:52 AM
Event ID:      25
Task Category: None
Level:         Warning
Keywords:      Classic
User:          N/A
Computer:      TLeonard-LT.HMR.local
Description:
Could not complete the operation. One or more parameter values are not valid.
Event Xml:
<Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
  <System>
    <Provider Name="Outlook" />
    <EventID Qualifiers="32768">25</EventID>
    <Level>3</Level>
    <Task>0</Task>
    <Keywords>0x80000000000000</Keywords>
    <TimeCreated SystemTime="2010-09-20T12:37:52.000000000Z" />
    <EventRecordID>4463</EventRecordID>
    <Channel>Application</Channel>
    <Computer>TLeonard-LT.HMR.local</Computer>
    <Security />
  </System>
  <EventData>
    <Data>Could not complete the operation. One or more parameter values are not valid.</Data>
  </EventData>
</Event>