Get-ProxyAddresses

function Get-ProxyAddresses
{
    Param(
    [Parameter(Mandatory=$true)] 
    [string[]]$username,
    [string[]]$domains = 'domain.com'
    )

    #Strip off any leading @ signs people may have provided. We'll add these later 
    $domains = $domains.Replace('@','')

    $ProxyAddresses = New-Object System.Collections.ArrayList
    foreach ($uname  in $username) {
        foreach ($domain in $domains ) {
            if ($ProxyAddresses.Count -lt 1) {
                $ProxyAddresses.Add( "SMTP:$uname@$domain" ) | Out-Null
            } else {
                $ProxyAddresses.Add( "smtp:$uname@$domain" ) | Out-Null
            }
        }
    }
    return $ProxyAddresses 
}

Get-ProxyAddress -username 'john.smith', 'james.smith' -domains 'domain.com','domain.net'
SMTP:john.smith@domain.com
smtp:john.smith@domain.net
smtp:james.smith@domain.com
smtp:james.smith@domain.net