How to Change the Sent Mail Ringtone on an iPhone

Open your iPhone's settings. Do so by tapping the grey gear icon on one of your Home screens (or in a folder named "Utilities").

Scroll to the third group of options and tap Sounds & Haptics. On some phones, this may just be Sounds.

Scroll to the "Sounds and Vibration Patterns" section and tap Sent Mail.

Select a sound. Doing so will set it as your new outgoing mail ringtone. For example, you might select Apex to set the "Apex" ringtone as your sent mail sound.