If question marks are being inserted into messages sent through Yahoo, AT&T, AOL, or Verizon servers, this is due to a server bug that causes 8BITMIME to be incorrectly returned as 8 BITMIME.
To work around this in Postbox, try these three steps in sequence, testing after each step (you don't want to do all three unless you need to):
1) Go to Preferences | Options > Composition > General and set the Character Encoding to "UTF-8."
2) If this still doesn't work, select the checkbox for Use the default character encoding in replies in that same panel.
3) Go to Preferences | Options > Advanced > General and select the checkbox for MIME encode 8-bit headers.
For an in-depth analysis of what's happening, please see this Mozilla article.