It sounds a lot to me like your bank is using a unicode font in the email and your apple product is converting that to some other non unicode font in the copy and paste.

The question marks arise when the font being used does not have a corresponding character for the underlying character number. It is becoming increasingly so that large institutions and corporations have a "corporate livery" they include in emails, including downloadable web fonts. Of course apple does not have the same fonts even as Windows, so it may be your bank has not even tested their email on apple desktops/Laptops, it really only represents a very small percentage of any market (about 7-8% worldwide), and how many of their Apple using customers copy and paste the email content anywhere to report it as a problem?