Data Source Validity Period dialog
When AY Mail loads a recipient list from an external data source, it doesn't dispose of it right away in case you resume sending within a short period of time.
You can choose for how long AY Mail keeps the list loaded. Longer delays will let you avoid importing the list over and over again, but impose a danger that the list will be outdated by several hours.
List remains valid for You can use any delay between 0 and 48 hours.