In the Customers folder of the reports there is a good report called Customer email list to select a purchase date range and get a list of all your customer email addresses

You can use this as a contact list to send out an email to all your customers during that selected time range

for this report, in the filter section you could choose last purchase date between and choose your start and end dates.

Once the report is generated you can go to the functions menu choose Export and choose Comma Delimited and Save the file with the name of your choice