The official docs say to put a 1 in front of the smtp address, so: 1 smtp.google.com
Set TTL to 1hr (3600 seconds)
https://apps.google.com/supportwidget/articlehome?hl=en&article_url=https%3A%2F%2Fsupport.google.com%2Fa%2Fanswer%2F174125%3Fhl%3Den&assistant_event=welcome&assistant_id=gsuitemxrecords-gixvmm&product_context=174125&product_name=UnuFlow&trigger_context=a