Right now that is only possible by accessing the bank. I know that mail is restricted because you can get unlimited amount of storage space that way.
But why not limit same-account mails to one or two at any time? If you limit mails to two, you can send two mails before you need to log in to another character and fetch these mails before you can send new ones.
This should be possible because there is already limit of 10 mails between players. Just copy that but make it one or two for same account mails.