You may be familiar with the feature of the Messaging app whereby you can send pre-written messages with just a couple of taps. This allows acknowledgement while not having to spend time typing a message. The same can be done with the built-in Email app (although not the Gmail app). To do this, open a message and tap the Reply icon. Then, tap the three-dots icon in the toolbar to open the menu.
Tap the last option in the menu, ‘Insert quick response’. You will see four messages; tap on the most appropriate and it will be copied into your message. Like email signatures, quick responses are specific to the account for which they’re being used. Open the Email app’s Settings menu, select an account and tap on the ‘Quick responses’ option.