ACK messages are not being sent

I'm using a TCP listener with "Basic TCP" transmission mode. The messages are receiving and processing on Mirth, just fine but for some reason the source sender is still marking the messages as "NAK". When I checked the response tab on Mirth, it says the following:



RECEIVED: HL7v2 ACK successfully generated.



MSH|^~\&|MIRTH|MIRTH|SENDER|SENFAC|2015020611525 3||ACK|20150206115253|P|2.3

MSA|AA|180543268013831



How can I check if these ACK messages are being delivered to the source sender? Or, is this a some sort of timeout issue?



Thanks

ACK messages are not being sent