It seems strange to me that a word that appears right on the page before me whenever I am typing an email is flagged as misspelled when I type it. Does the spell check function draw from some antiquated database that lacks this basic word?
The spellcheck that marks words spelled incorrectly with a red squiggly line comes from your browser. You'll need to update that dictionary if you want words like "inbox" marked in the good and clear.
Gmail's spellchecker doesn't kick in until you hit the "Check Spelling" link upper-right to the email body. Any words spelt wrong will then be highlighted on a yellow background.