>I have multiple family members who have healthy memory, but can't accurately remember their email address everytime: the localpart, the domain, the syntax, everything.

But you can't do anything about that except asking them to validate their address with an email.

If you can catch 50% of user errors with some complex regex, but the other 50% such errors are uncaught, is that of any benefit during sofware design? No, because you still have to solve that problem, probably with email validation by code. You have reduced your workload by 0%, you just split it into 2 parts (unnecessarily).