One customer calls our service his "apologizer". He occasionally needs to send mail to selected customers to explain a glitch in their ordering process. His messages are personalized to include customer name, order number, order date, amount of the transaction, etc., taken from his order processing system as needed. Because each contact list remains on the maillists.com server, he can send a followup message a few days later to the same people to assure all transactions have been corrected properly. The bounce report shows any emails which could not be delivered.
Visitor followup system
This user is a mega-church in Southern California. They use the Google-spreadsheet option. Each Sunday they use tablets to record visitor information such as name, address, email address, area of interest, etc. into a Google spreadsheet. Then each Monday, a custom app we've written sends a customized followup message to each visitor to address the specific interest they expressed during their visit.
We enjoy providing this service. Some Christian missionaries have hundreds of contacts, but organize them by "supporters", "prayer warriors", "church leaders", etc. They can maintain each group separately on the server, then schedule emails as needed, to specific groups or to their entire database. Addresses which might appear in more than one group are merged, so each gets only one copy of any message.