Supporters are given the option to opt into your mailing list as part of the process of completing a transaction through Flipcause. If that mailing list box is checked, two things happen:
1. They receive an email thanking them for joining the mailing list. This email can be customized by going to Campaigns > Edit this Campaign > Campaign Settings > Receipt Settings > Mailing List Custom Message.
2. They show up in the Mailing List Sign-up Report. This can be pulled by going to Reports > Transaction and Activity > Set Date Range > Report Type=Mailing List Sign-up > Generate Report.
You can take that report you just generated and export it as a CSV file. You can do that by clicking Custom Report > Select the fields you want to include > Send report to your email as a CSV file > Get Report. You can then take the CSV file you receive in your inbox and import it into your email marketing tool from there.