You can tweak your gmail account to add a customized REPLY-TO on every mail you send via this account:
- Open your gmail account settings.
- Select Accounts and Import
- Go to Send as
- Click Add another address
- Type in your name and the reply-to address you wish to use and select Use as alias
- Press Next step
- Specify gmails own smtp ans imap servers as servers. Use your gmail login and password.
On your iPhone:
- Add the new account. Do not use the default GMail wizard.
- Go to Settings -> ‘Mail, Contacts, Calendar’ -> Accounts
- Select THIS account.
- Select ‘Account’
- Go to section ‘IMAP-ACCOUNTINFO’
- select ‘E-Mail’
- Add your REPLY-TO address and delete the other.