How to count messages sent and received for each mailbox in a mailbox database

I have a customer who has standard journaling configured for a single mailbox database. They have a need to generate a quarterly report on the number of messages that are journaled with a breakdown of messages by mailbox. I generated a script that works for a single mail recipient. The script follows:

$StartDate = Read-Host -Prompt 'Enter the start date (m/d/yyyy)'
$EndDate = Read-Host -Prompt 'Enter the stop date (m/d/yyyy)'
$ReportRecipient = Read-Host 'Enter the email address to receive this report: '

$msgsreceived = Get-TransportServer | Get-MessageTrackingLog -Recipients "journalmailbox@domain.com" -resultsize unlimited -start $StartDate -end $EndDate -EventID Receive

$msgsreceived | Select-Object sender,timestamp,messagesubject  | ft -AutoSize | Out-file C:\temp2\GetMessageCountResults.csv

$count = "Number of messages" + $msgsreceived.count
$count | Out-file -Append C:\temp2\GetMessageCountResults.csv

Send-MailMessage -SmtpServer servername -to $ReportRecipient -From servername@domain.com -Subject "Journal message count from $startDate to $EndDate" -Body "Attached is the Journal Message count" -Attachments C:\temp2\GetMessageCountResults.csv

Now that I know that the script works on a single mailbox, I need to get it to work on all mailboxes in a mailbox database so the script doesn't need to be adjusted each time a mailbox gets added or removed from the database.  I tried to include a foreach-object statement but it produced an error.  The modified script looks like this:

$StartDate = Read-Host -Prompt 'Enter the start date (m/d/yyyy)'
$EndDate = Read-Host -Prompt 'Enter the stop date (m/d/yyyy)'
$ReportRecipient = Read-Host 'Enter the email address to receive this report: '

$msgsreceived = Get-TransportServer | Get-MessageTrackingLog -Recipients "mrwj@mvsb.com" -resultsize unlimited -start $StartDate -end $EndDate -EventID Receive

$msgsreceived | Select-Object sender,timestamp,messagesubject  | ft -AutoSize | Out-file C:\temp2\GetMessageCountResults.csv

$count = "Number of messages" + $msgsreceived.count
$count | Out-file -Append C:\temp2\GetMessageCountResults.csv

Send-MailMessage -SmtpServer nhmbexch2 -to $ReportRecipient -From nhmbexch2@nhmutual.com -Subject "Journal message count from $startDate to $EndDate" -Body "Attached is the Journal Message count" -Attachments C:\temp2\GetMessageCountResults.csv

I can use some help with this.