You can change or add another email address to your iOS mail settings. Set a primary email address in your mobile device settings under Settings – Mail – Default account.

You can also switch between more email accounts while sending invoices – click here to learn more.
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Invoice Maker uses Gmail as the mail app of choice if there are multiple installed.
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If you'd like to keep the Gmail app on your phone and still send invoices from another email address, it's necessary to add your email address to Gmail. Take a look here for more information.

You can use SMTP to use your own email. Take a look here.