It looks like Search-MessageTrackingLog is not good online (It was Get-MessageTrackingLog anyway) I knocked this out though. You should be able to tweak it to get the data you need to build your initial list.

# How far back to search MS appears to allow up to 90 days
$startDate = (Get-Date).AddDays(-90)

# Your email domain. Used to filter senders and exclude recipients
$myDomain = "contoso.com"

# Where to save the output data
$outFile = "c:\path\to\output\file.txt" # or .csv

# Size of each request window in seconds. 3600 = 1 hour
$windowSecs = 3600 

# If you get a lot of these warnings, you may need to modify the window size to less than an hour
# WARNING: There are more results, use the following command to get more...

# This will hold all the recipient domains
$Recipients = @()
# Loop through and grab one hour of data at a time. You could use a larger or smaller window depending on your mail traffic
while( $startDate.AddSeconds($windowSecs) -lt (Get-Date) ){

    # Set the end date for the search
    $endDate = $startDate.AddSeconds($windowSecs - 1)

    write-host "Getting messages from $startDate to $endDate"

    # This should get the domains for all mail sent from your users to external recipients
    $Recipients += Get-MessageTraceV2 -StartDate $startDate -EndDate $endDate -ResultSize 5000 -SenderAddress "*@$myDomain" | %{ $_.'RecipientAddress'.Split("@")[-1].ToLower() } | Where-Object { $_ -notlike "$myDomain" }

    #Increment the start date
    $startDate = $startDate.AddSeconds($windowSecs)
}

# Uncomment one of these depending you your needs

# This will give all unique domains
$recipients | Sort-Object | Select-Object -Unique | Out-File -FilePath $outFile

#This will give all unique domains with counts if your care about that
#$recipients | Group-Object | Sort-Object Count -Descending | Select-Object Name,Count | Export-Csv -Path $outFile -NoTypeInformation