When the post-by-email system is implemented, I assume people are going to have to remember to surround code blocks with the [code] tags in those if they want indented blocks to appear correctly on the forum, too? If so, you'll have an even harder time of getting email users to remember to put these in when posting, I think. When posting a message via the website you can always remind people either on the same page or via the popup help, but when people are posting by email they aren't used to having to use a proprietary set of tags just to be able to format their post correctly.

Having said that I can't see why replacing four consecutive spaces with a [t] tag would be a problem anywhere, really, though you'd probably want to re-replace the [t] with spaces on outgoing emails to avoid confusion.