Robocopy script /w Powershell + Mail report for failure - powershell

Question

I'm studying for Network\System Admin certification (MCSE is in it) and among other things we started to delve into powershell scripting.
At any rate my instructor gave me a challenge to do as follows:
I need to create a PS script which will run Robocopy tool in order to mirror a folder from point A to Point B and that script should run as a schedule task (/w Task Scheduler).
If robocopy have encountered any errors the script should send a mail via the Company's exchange mail server to the Admin in charge of the process.
I'm not Scripting savvy of any sort so I had to sit and learn the syntax, command etc before I could start working.
Here is what I got so far:
# PowerShell Robocopy Script V.1 | Liron Ben-David
# Variables
$Source ="C:\Source\"
$Destination = "C:\Destination\"
$ExitCode = "Null"
#Copying Process
Robocopy $Source $destination *.* /mir /r:100 /w:10
# Return Code Section
if ($LASTEXITCODE -eq 0) {$ExitCode = "Succeeded, Code: $LASTEXITCODE" }
elseif (($LASTEXITCODE -gt 1) -and ($LASTEXITCODE -lt 17)) {$ExitCode = "Failure, Code: $LASTEXITCODE" }
else {$ExitCode = "No Return Code"}
Write-Host $ExitCode
## Failure Report
if ($ExitCode -gt 1) {Send-MailMessage -to "Admin (Admin#Domain.com)" -From "Machine-Robocopy1 (M1RC#domain.com)" -Subject $ExitCode -SmtpServer mail.domain.com}
else {exit}
basically if there is any kind of error in the process the script supposed to send an email with the script error code as the "Subject" of the message.
First of all:
Does this script contain any embarassing mistakes I did and which I should avoid in the future?
Secondarily:
I never used the CLI to send email messages, using the Send-Mail CMDLET I should have used a credentials switch but according to what I've read\watched it seems it will pop up a window for input.
Considering I'm setting this script for a Domain Environment wouldn't the SSO feature will kick in when I try to send the message?
EDIT:
Also noticed I mixed the error codes so I need to re-calibrate the error code lines.
EDIT 2: Version 2 bsaed on your help,
# PowerShell Robocopy Script V.2 | Liron Ben-David
# Variables
$Source = "C:\Source\"
$Destination = "C:\Destination\"
$ExitCode = "Null"
$ReturnCodeValue = "Void"
$Log = "C:\RoboLogs\RobocopyLog.txt"
#Copying Process
Robocopy $Source $destination *.* /mir /r:100 /w:10 /UniLog:C:\RoboLogs\RobocopyLog.txt /v
# Return Code Section
if (($LASTEXITCODE -eq 0) -or ($LASTEXITCODE -eq 1) -or ($LASTEXITCODE -eq 3) -or ($LASTEXITCODE -eq 5) -or ($LASTEXITCODE -eq 6) -or ($LASTEXITCODE -eq 7)) {$ExitCode = "Succeeded, Code: $LASTEXITCODE" }
elseif (($LASTEXITCODE -eq 2) -or ($LASTEXITCODE -eq 4)) {$ExitCode = "Error, Code: $LASTEXITCODE" }
elseif ($LASTEXITCODE -gt 7) {$ExitCode = "Failure, Code: $LASTEXITCODE" }
else {$ExitCode = "No Return Code"}
$ReturnCodeValue = $LASTEXITCODE
# Debug Section
#Write-Host "Did the script completed succesfuly:" $?
#Write-Host "Script Exit Code:" $ExitCode
#Start-Sleep -s 4
## Failure Report
if (($ReturnCodeValue -gt 7) -or ($ReturnCodeValue -eq 2) -or ($ReturnCodeValue -eq 4)) {Send-MailMessage -to "Admin (Admin#Domain.com)" -From "Machine-Robocopy1 (M1RC#domain.com)" -Subject $ExitCode -SmtpServer mail.domain.com –Attachments $Log -UseSsl}
else {exit}