a universal forwarder will request to resolve XXXXXX (DNS) and it may get an IP address of the indexer that is no longer available
in that case, it doesn’t have another address to try, so what will it do
You should check out the Protect against loss of in-flight data section of the forwarding manual, as well as the Persistent Queues section of the Getting Data In manual. TL;DR: For some inputs, Splunk has a configurable queue where it will store data for when forwarding can resume. For other types of inputs, it will stop following the input until the pipeline is clear again.