If you need to export a list of user mailboxes (because your boss is making you or you simply have nothing better to do), it is quite an easy task if you have Exchange 2007 and up because of PowerShell snapins.
You can just fire up PowerShell and import the Exchange 2010 Module.
Add-PSSnapin Microsoft.Exchange.Management.PowerShell.E2010
Then:
Get-Mailbox -ResultSize Unlimited | Get-MailboxStatistics | select-object Database, DisplayName, TotalItemSize, TotalDeletedItemSize | Sort-Object Database
Or you can export to CSV like so: Get-Mailbox -ResultSize Unlimited | Get-MailboxStatistics | select-object Database, DisplayName, TotalItemSize, TotalDeletedItemSize | Sort-Object Database | Export-Csv .\Mailboxes.CSV -NoTypeInformation
But what if you’re still in survival mode with Exchange 2003?
You can use this script should you need to extract the list of user mailboxes from one or more Exchange 2003 servers.
This script will cough-out the following Fields.

• ServerName
• StorageGroupName
• StoreName
• MailboxGUID
• MailboxDisplayName
• LegacyDN
• Size
• TotalItems
• AssocContentCount
• DeletedMessageSizeExtended
• StorageLimitInfo
• LastLoggedOnUserAccount
• LastLogOnTime
• LastLogOffTime
• DateDiscoveredAbsentAbsentDaysInDS

This script reads the list of servers to be queried from a file called ServerList.ini. Be sure to create this file and populate it with the server names before running this script.
It runs in this order:
1. Read list of server from ServerList.ini 2. Query each servers and extract information 3. Save information to “ExchangeMBX.txt”

'==========================================================================
'
' NAME: ExtractMBXInfo.vbs
'
' AUTHOR: june.castillote@gmail.com
' DATE  : 10/01/2011
'
' COMMENT: This is for extracting the list of mailboxes from specified servers in the "serverlist.ini" file.
' FILES : 1. ExtractMBXInfo.vbs - main script
'          2. ServerList.ini - file containing the list of servers for the query.
' USAGE  : cscript ExtractMBXInfo.vbs
'==========================================================================
Option Explicit

On Error Resume Next

Dim t1, t2, t3, d1, d2, d3

Dim strComputer

'==========================================================================

Dim FileToWrite, FileToRead, fsoWrite, fsoRead

Set fsoWrite = CreateObject("Scripting.FileSystemObject")
Set fsoRead = CreateObject("Scripting.FileSystemObject")

Set FileToWrite = fsoWrite.CreateTextFile("ExchangeMBX.txt")
Set FileToRead = fsoRead.OpenTextFile("ServerList.ini")

Dim objWMIService
Dim colItems
Dim objItem
Dim i

'Header row
FileToWrite.WriteLine "Server" & vbTab & "Storage Group" & vbTab & "Mail Store" & vbTab & "Mailbox GUID" & vbTab & "Display Name" & vbTab & "LegacyDN" & vbTab & "Size" & vbTab & "Item Count" & vbTab & "Associated Content Count" & vbTab & "Deleted Message Size" & vbTab & "Date Absent" & vbTab & "Storage Limit Level" & vbtab & "Last LogOn Account" & vbtab & "Last LogOn Time" & vbTab & "Last LogOff Time"

'Iterate through the list of servers
Do While Not FileToRead.AtEndOfStream
     strComputer = FileToRead.ReadLine()
     WScript.Echo Now & " : Connecting to " & strComputer
     Set objWMIService = GetObject("winmgmts:" _
        & "{impersonationLevel=impersonate}!\\" & strComputer & _
            "\ROOT\MicrosoftExchangeV2")

    WScript.Echo Now & " : Running Query on " & strComputer
    Set colItems = objWMIService.ExecQuery _
    ("Select * from Exchange_Mailbox")

    For Each objItem in colItems
        If objItem.LastLogOnTime <> "" Then
            t1=WMIDateStringToDate(objItem.LastLogonTime)
        Else
            t1 = ""
        End If

        If objItem.LastLogOffTime <> "" Then
            t2=WMIDateStringToDate(objItem.LastLogOffTime)
        Else
            t2 = ""
        End If

        If objItem.DateDiscoveredAbsentAbsentDaysInDS <> "" Then
            t3=WMIDateStringToDate(objItem.DateDiscoveredAbsentInDS)
        Else
            t3=""
        End If            

        FileToWrite.WriteLine objItem.ServerName & vbTab & objItem.StorageGroupName & vbTab & objItem.StoreName  & vbTab & objItem.MailboxGUID & vbTab & objItem.MailboxDisplayName & vbTab & objItem.LegacyDN & vbTab & objItem.Size & vbTab & objItem.TotalItems & vbTab & objItem.AssocContentCount & vbTab & objItem.DeletedMessageSizeExtended & vbTab & t3 & vbTab & objItem.StorageLimitInfo & vbTab & objItem.LastLoggedOnUserAccount & vbTab & t1 & vbTab & t2
    Next
Loop
WScript.Echo Now & " : End - Saved to ExchangeMBX.txt"

'To convert WMI time to Standard time format
Function WMIDateStringToDate(dtmInstallDate)
    WMIDateStringToDate = CDate(Mid(dtmInstallDate, 5, 2) & "/" & _
    Mid(dtmInstallDate, 7, 2) & "/" & Left(dtmInstallDate, 4) _
    & " " & Mid (dtmInstallDate, 9, 2) & ":" & _
    Mid(dtmInstallDate, 11, 2) & ":" & Mid(dtmInstallDate, _
    13, 2))
End Function

FileToRead.Close
FileToWrite.Close
Set FileToRead = Nothing
Set FileToWrite = Nothing