Finding and Fixing IMCEAEX delivery issues

After migrating email from Office 365 back on premise we ran in to a lot of failed deliveries based legacy email addresses. Clearing out cached email address from the users however the issues kept appearing.

Run the following on the exchange server to find what all emails have been failing

Get-TransportServer -identity "server" | Get-MessageTrackinglog -EventID FAIL -Start (Get
-Date).AddDays(-15) -ResultSize Unlimited | Where {$_.Recipients -match "^IMCEAEX*"} |fl

If you want to export the recipients to a csv run the following

Get-TransportServer -identity "server" | Get-MessageTrackinglog -EventID FAIL -Start (Get
-Date).AddDays(-15) -ResultSize Unlimited | Where {$_.Recipients -match "^IMCEAEX*"} |Select @{l="Recipients";e={$_.Recipient
s -join " "}} |Export-csv C:\temp\undeliv.csv

Then open powershell and use the following script to convert the IMCEAEX to an X500 address

Param(
    [string]$IMCEAEX
)

Write-Host ""
Write-Host -ForegroundColor Magenta "**********************************************************"
Write-Host -ForegroundColor Magenta "****** Convert your IMCEAEX NDR to an X.500 address ******"
Write-Host -ForegroundColor Magenta "**********************************************************"

If ($IMCEAEX -eq "") {
    Write-Host -ForegroundColor Yellow -NoNewline "`nPaste your IMCEAEX NDR string here: "
    $IMCEAEX = Read-Host
}

If($IMCEAEX.Substring(0,7) -ne "IMCEAEX") {
    Write-Host -ForegroundColor Red "`nSorry, your IMCEAEX string must begin with IMCEAEX`n" 
} Else {
    $X500 = $IMCEAEX.Replace("IMCEAEX-","X500:").Replace("_","/").Replace("+20"," ").Replace("+28","(").Replace("+29",")").Replace("+2E",".").Replace("%3D","=").Split("@")[0]
    Write-Host 
    Write-Host -ForegroundColor DarkCyan "Your converted X.500 address is: `n" 
    Write-Host -ForegroundColor Green $X500 `n
    Write-Host -ForegroundColor DarkCyan "Here is the Set-Mailbox command to add the X.500 address to a user (change the Identity attribute accordingly): `n"
    Write-Host -ForegroundColor Green "Set-Mailbox -Identity first.last@domain.com -EmailAddresses @{add=`"$X500`"}" `n
    Write-Host -ForegroundColor Yellow "Done!`n"
}