https://apidocs.api19.com/sms-mms/receive-delivery-receipts
Receive Delivery Receipts | API Docs
Receive a SMS message to your system
delivery receiptsreceiveapidocs
https://knowledgebase.smsglobal.com/delivery-receipts-via-smpp-smsglobal-help-center
Delivery Receipts via SMPP
SMS Global Knowledgebase
delivery receiptsviasmpp
https://www.readnotify.com/default.asp/from/1netcen
Certified email with delivery receipts, silent tracking, proof-of-opening history, security and...
Users get free return email notifications, and/or SMS/ICQ instant messages when email they've sent gets opened, and can track their emails reading history