Reversely, with an iPhone or Android you can use your computer accounts (gmail etc...) on your mobile phone.
Meaning that you better never use a proprietary-mobile dependent address, but only your computer mail on your mobile (thus you can change mobile - e.g. from iPhone to Android [just reading the 1.5 year future ;-] without the well known mail address dependency problem)... provided that your mobile allows an IMAP connection (like iphone android ...).
If you don't have a choice, I suggest you ask directly Softbank:
this is the
English support page - think there is a way, check directly with SB.