You've got it! US vs UK English. You could try this to not flag the spellings, although the recipient is likely to notice them also:
In Word versions prior to Word 2007:
1. Click the Tools->Options menu item.
2. Click the Spelling and Grammar tab.
3. Check the Hide spelling errors in this document checkbox.
4. Check the Hide grammatical errors in this document checkbox.
5. Click OK.
In Word 2007 and 2010:
1. In Word 2007, click the round Office button in the upper-left corner. In Word 2010, click the Files tab on the Ribbon.
2. In Word 2007, click the Word Options button. In Word 2010, click Options.
3. Click Proofing.
4. Check Check Spelling As You Type.
5. Check the Hide Spelling Errors In This Document checkbox.
6. Check the Hide Grammar Errors In This Document checkbox.
7. Click OK.
Hope that helps.