Is congratulating Muslims one another via mobile messages on religious holidays, such as the Noble Prophet's Birthday (Mawlid an-Nabawi), considered an innovation (bid‘ah) or a sin?

Messages exchanged on occasions are of two types: First, those sent on legitimate Islamic holidays and occasions for congratulation or to remind of a specific act of worship. This is permissible if its content is sound. Second, those sent on innovated or illegitimate holidays, such as the Prophet's Birthday (Mawlid al-Nabawi) or the holidays of disbelievers. This is forbidden because it is a congratulation for a religious innovation or an imitation of disbelievers. The Prophet, peace and blessings be upon him, said: "Whoever calls to guidance will have a reward like the rewards of those who follow him, without diminishing anything from their rewards. And whoever calls to misguidance will bear a sin like the sins of those who follow him, without diminishing anything from their sins."