Save this to a ps1 and use EMS as an admin and bosh!
$contacts = Get-MailContact -Filter {ExternalEmailAddress -like '*WRONGDOMAIN.com*'} </i><br />
<i>$contacts | foreach-object {$_ | set-mailcontact -externalemailaddress ($_.externalemailaddress.proxyaddressstring -replace "WRONGDOMAIN","RIGHTDOMAIN")}