If you are looking for a way to change the Default Email Address on iPhone, it is likely that your Email Address has changed, you have added multiple Email Addresses to iPhone or you do not want to use your Apple ID linked Email address to send and receive Email messages.
The later case occurs due to the way iPhone prompts users to provide an Email Address during the initial setup process, which automatically becomes your Apple ID and the Default Email Address for sending and receiving emails from iPhone.
Luckily, the Mail App on iPhone provides the option to add multiple email addresses and designate any of these Email Addresses as the default Email Address for sending and receiving Emails from iPhone.
Change Default Email Address for Mail App on iPhone
In order to change the Default Email Address on iPhone, the first step is to add your New Email Address to the Mail App on iPhone, without removing your existing Email Account.
Once the New or Alternate Email Address is available in the Mail App, the next step is to designate this Email Address as the Default Email Address for sending and receiving Emails from iPhone.
Step 1. Add New Email Address to Mail App on iPhone
As mentioned above, the first step is to Add your New Email Address to the Mail App on iPhone.
Note: You can skip this step, if a New or additional Email Address is already available in the Mail App on your iPhone.
1. Go to Settings > Apps > Mail.
2. From the Mail screen, go to Mail Accounts and tap on Add Account.

3. On Add Account screen, tap on choose from a list link.

4. On the pop-up, select the Email Account Type (Google, Outlook, etc.) that you want to Add.

5. On the next screens, enter your New Email Address and Password to login to your Email Account.
With these steps, you have added your New Email Address to the Mail App, which in the next step can be designated as your primary or the default Email Address on iPhone.
Step 2. Change Default Email Address on iPhone
Follow the steps below to designate the New Email Address that you just added to the Mail App as the Default Email Address for sending and receiving Emails from your iPhone.
1. Go to Settings > Apps > Mail.
2. On the Mail screen, scroll down and tap on Default Mail App.

Note: The “Default Mail App” option will be available, only if there is an alternate or second Email Address in the Mail App.
3. On the next screen, select the Email Account that you want to set as the Default Email Address on your iPhone.

4. After this, you should see the change in Default Email Account. If not, restart your device and this will reflect the change in Default Email Address.
After these two steps are completed, your iPhone by default will be using your New Email Address for sending and receiving Email Messages from the Mail App.