This might work. (The first part works, but I haven't tested the remove command).
Just in case you have an account with more than one mx360 address.
Get-ADUser -Properties proxyaddresses -Filter {ProxyAddresses -like '*mx360@contoso.com'} |
ForEach { # Account may have more than one email address in scope so need to loop through each one
ForEach ($proxyAddress in $_.proxyAddresses) {
If ($proxyAddress -like '*mx360@contoso.com') {
# Write-Host $proxyAddress
Set-ADUser $_.SamAccountName -Remove @{ProxyAddresses=$proxyAddress}
}
}
}