Vodacom Prepaid Email Address

I recently changed internet service providers from Vodacom Mobile to another, however in order to retain my Vodacom email address I had them convert my expired contract to a prepaid SIM, and everything has gone relatively smoothly.

Does anyone know what are Vodacom's rules for keeping this email address valid ? i.e Do I need to top up with data every so often, do I need to make a chargeable call or send an sms occasionally (problem - sim is in a data only modem). Is it just enough that the email address is active sending and receiving mail ? Can I just use it forever and do none of the above (doesn't sound likely).

Have searched everywhere possible and don't have the strength to ask the VC call centre.

Thanks !