There are three main options to test the content of your autoresponder.  In all of the following scenarios you will need to apply an initial successful sustaining payment. Then, instead of waiting one month for the next payment to trigger, you can modify/edit the sustaining payment and change the next payment date to the next day. Thus all scenarios will take one day to fully test.

First, if you have a Dev Site, would be to create the desired content in your Dev Environment and from this there is an SDP (CCOM_TRANS_MODE:) that can be set to control how the transactions are treated. This will allow you to create a sustaining gift in the Dev Environment and have it succeed as the SDP is defaulted to 'Test_Good' mode which will simulate a successful transaction no matter what. This SDP is a Blackbaud only SDP but once you have established the content and the successful sustaining gift Blackbaud Support can change the SDP to 'Test_Decline' and this will make all transactions fail. Blackbaud Support can make these changes for you as the access of this SDP is restricted.

Second would be to test this by adding the same content from the failed sustaining attempt autoresponder to the initial thank you for sustaining giving auto responder. This is not a perfect representation but it should be close depending on the content you are testing.

Third would be to have a card that you can guarantee will be able to fail on the second attempt to process. This can usually be done with a prepaid credit/debit card, though this will only work for some prepaid debit cards as some do not allow online purchases. The process of this would be to buy a card that only has $10-25 on it and make the initial payment for the full amount.  This will make sure that the second attempt will fail and the proper autoresponder for the failed attempt should be sent out.