In the event that an ISP or digital services provider wishes to cease providing email service, they could be required to provide minimal access to enable updated provision of either a forwarding email address (which the provider would forward mail to), or an MX to redirect the sender to use. This would avoid the need for a big central "porting server", and retain the simplicity of doing a DNS lookup for one or more current MX records.
Given spam challenges, the former is likely unattractive (who wants to use their own IP ranges to relay potential spam to former users?), but the latter could likely work, or be made to work.
I realise the MX approach doesn't quite work, and you'd likely need to relay the email, but this is more akin to how the phone number porting system works - you ask the number block owner on each call, and they can either accept the call, or point you towards the correct destination network.
This topic is of indirect relevance and interest to the UK telecoms regulator [1], since many households rely on an email address from their ISP, which could become a barrier to switching, or result in long-term extractive pricing from users who have no real choice other than to pay a former ISP over the odds for email service to retain an old address.
[1] https://www.ofcom.org.uk/about-ofcom/latest/features-and-new...
In your example, anyone who wants to legally run an email hosting business.
What could possibly go wrong?
All people sending email will be required by law to first consult the porting server to determine where to send email. It could be as simple as an http endpoint. Total development costs and running costs could be pretty low - low enough that the government can argue it is more than paid for from increased tax revenue from mail providers.