You can no longer send bulk emails through Selling Manager Pro but there is a workaround by doing it through eBay Singapore.

Login to ebay.com.sg

In your My eBay under the Account tab click on Subscriptions and subscribe to the free Selling Manager.

Select the buyers that you wish to email from your Sold section

Click on the Other Action button and select Email Buyer

Select which Email Template to use or create a new one

If you are sending regular bulk emails you can have Templates setup for different purposes and saved ready to use.