Your default email address is your first email you have set up on your iPhone or iPad. It could be your personal or professional email that you access every day. However, you can use multiple email IDs on your iOS device.
Once you have added more than one email to your iPhone or iPad, you can choose which one to set as default email address. Please note that in earlier iOS versions, it was enough to select default email id from the Settings; now, you have to follow a few more steps.
How to Change Default Email Address for Mail App on iPhone and iPad
Step #1. Launch Settings app on your iPhone/iPad → Scroll down and tap on Mail, Contacts, Calendars.
Step #2. Again scroll down and tap on Default Account from under the MAIL section.
Step #3. Select any account you want to set as Default Account.
Step #4. Scroll down a bit and tap on Default Account from under CONTACTS section.
Step #5. Tap on the Account you want to set as Default Account.
Step #6. Tap on Default Calendar from under the CALENDARS section.
Step #7. Tap on the email ID you have set as Default Account from MAIL and CONTACTS sections.
Now you default account is changed; whenever you send an email from the Mail app on your iPhone or iPad, the receiver will get your emails from the Default Account only. Once you set an email as Default Account, it is advisable to reboot your iPhone or iPad.
Note that some users confuse the change of Default Email address with the change in email address associated with Apple ID. But believe me these two things are completely different.