Is increasing the Hotmail e-mail size to 250MB, using a method that involves changing data that might include credit card numbers of other subscribers, considered stealing or deception?

It is not permissible to increase the email size in a way that is prohibited by the service provider company, because that is considered deception and a violation of the terms. The Prophet, peace and blessings be upon him, said: "Whoever cheats is not from me," and he said: "Muslims are bound by their conditions concerning what is permissible." One must repent to Allah from this, and if it becomes clear that there is theft involved, then the right must be returned to its owners if possible, otherwise it should be spent on the welfare of the Muslims.