How can I configure the iPhone to send/receive mail through Gmail, but using a different e-mail address?

I have a pobox.com account whose address I use as my main e-mail address and that forwards my messages to a Gmail address.

Eventough that is the address I tell my friends and coleagues, when I send e-mail from my iPhone it uses the default gmail.com address, and so people start to pick that as my e-mail address.

How should I configure a Gmail account in the iPhone that uses my pobox.com address for sent messages, instead of the gmail.com address? How should I set up storage of sent/junk/trash messages? How will Gmail archiving work?

This worked for me

Assuming you have already setup the alternate account in your gmail

• Remove the gmail account from your iphone
• Add new mail account
  
  • Settings > Mail > Add Account > Other
  • Enter standard gmail id and password in New Account screen
  • Enter standard gmail id and password in IMAP and SMTP screens (imap.gmail.com, smtp.gmail.com)
  • After account is setup, under "IMAP Account Information" change address field to the desired account (Note that the "Incoming Mail Server" settings will remain as gmail info, this is the difference between creating the account manually and using the automatic gmail setup)

Enjoy

I've found a nice step-through, and it does what I need, but messages I delete in my iPhone continue appearing in the Gmail Inbox in the browser because the "inbox" label isn't removed.

http://5thirtyone.com/apple/how-to-proper-gmail-imap-for-iphone-apple-mail/