Your data is absolutely safe!

No user data or any other data related to Do It Later will ever be sold or made available to third parties. With Do It Later, what happens on your device stays on your device.

Do It Later does not send any data, anywhere, unless it is requested by the user (e.g. sending an automatic SMS or E-mail)

This question also related to the following questions:
Why does the app not working sometimes?
Why does the app send message with a delay time?
Why does the app only send message once I turn screen on or open the app?

Many phone manufacturers customize the android operating system on their phones to prevent the app to run from background. 
To get it working without restriction, we recommend checking out the links below and follow its instruction to
enable auto-start permission for Do It Later on your device

Pixel:  https://dontkillmyapp.com/google
Huawei:  https://dontkillmyapp.com/huawei
Samsung:  https://dontkillmyapp.com/samsung
Oneplus:  https://dontkillmyapp.com/oneplus
Xiaomi:  https://dontkillmyapp.com/xiaomi
Oppo:  https://dontkillmyapp.com/oppo
Meizu:  https://dontkillmyapp.com/meizu
Asus:  https://dontkillmyapp.com/asus
Realme:  https://dontkillmyapp.com/realme

Do It Later send text to multiple recipients individually, not a group message.

Normal user can add maximum 3 recipients for one message. For premium user, there is no limit on the number of recipients. But please notice that you should avoid adding many recipients for one message, split it into some messages instead.

☀ Best practice: Suppose that you want to send a message to 90 recipients, It is better if you create 3 messages separately, each one with 30 recipients and schedule them with a little bit different time.

Some devices (e.g. Huawei) is customized to allow only Default SMS app to write data into SMS database, it is a reason why you didn’t see completed messages on your default SMS app.

Gmail and Twitter scheduling feature is required internet connection to send the message, others don’t require internet connection.

No. Do It Later can only be used from Android devices.