Aqua Mail uses thespell checker built into the system. This has been possible since Android 4.0. You can enable it in the device’s system settings, under Languages and Input. Once this is done, spell checking will be automatically enabled in the message compose screen.

Note: If the setting is not there – the feature may have been removed by your Android device manufacturer from their flavor of Android system software. It may also be available by another name. Contact your device manufacturer's support for more help.