Using Set-ADUser to change an AD account outside current forest

So I’m trying to a bulk change to users’ proxyaddresses. The issue is that these users reside in different AD forests and I keep getting this error:

Set-ADUser : Cannot find an object with identity: ‘xxx’ under: ‘DC=loc1,DC=dc,DC=company,DC=com’.

which makes sense because these users are spread throughout these forests:

I able to use the Get-ADUser properties of the user in the other forests using this command:

$User = get-aduser -filter {UserPrincipalName -eq $upn} -Server FQDN:3268

but I can’t seem to figure out how to get the set-aduser command to check all forests for the user. It seems to be only checking the forest that my AD account currently resides in (dc=loc1).

Any help would be appreciated. I should warn you that I’m new to ps, so if you could show the actual code, that would help tremendously.

My PS knowledge is also low…Few thoughts:
A loop to check all forests
Use a credential that is global to run the script
Try select-object and use the object-guid with get-aduser and not use the upn

I don’t have an AD environment to test, but something like this should work.

$forest = ''.Trim() -split "`n"

# List of users
$user = Import-Csv userlist.csv
# Check each server for users then make a change
foreach ($f in $forest){
    foreach ($u in $user){
        $instance = Get-AdUser -Server $f -Properties proxyaddress -Filter {UserPrincipalName -eq $u}
        $instance.ProxyAddress = "yournewaddress"
        Set-ADUser -Instance $instance

Thanks Random, that worked perfectly. I had to change the $forest to just actual text, it didn’t like the split formatting for whatever reason.

May I ask why would putting the cmdlet in a variable work over just running the straight cmdlet?

$Forest = Get-ADForest |select domains

$user = Import-Csv c:\conflicts.csv
ForEach ($u in $User){
foreach ($f in $forest){
    $upn = $u.UPN
    $sn = "smtp:"+$u.OldEmail
    $obj = Get-AdUser -Server $f -Properties proxyaddresses -Filter {UserPrincipalName -eq $upn}
    If ($obj -ne $null){
    $obj.ProxyAddesses += $sn
    Set-ADUser -Instance $obj

Ok, I had an epiphany and found a better way. Instead of going through all the domains trying to find the correct domain, why not just get the domain directly? Here is the updated code. Make sure to update the with your AD Global Catalog FQDN:

$user = Import-Csv C:\proxytest.csv
ForEach ($u in $User){

    $upn = $u.UPN
    $sn = "smtp:"+$u.OldEmail

    #this searches for the user and the OU they are listed in
    $dname = Get-AdUser -Server :3268 -Properties canonicalname -Filter {UserPrincipalName -eq $upn}
    #this extracts the OU
    $ou = $dname.CanonicalName
    $pos = $ou.IndexOf("/")
    $dn = $ou.Substring(0, $pos)

    #in AD, you need to specify the actual domain they are listed in
    $obj = Get-AdUser -Server $dn -Properties proxyaddresses -Filter {UserPrincipalName -eq $upn}
    If ($obj -ne $null){
    $obj.ProxyAddresses += $sn
    #removes the proxyaddress

    Set-ADUser -Instance $obj
    write-host "User does not exist in AD: "$upn}