I don't feel there's any real benefit to spending time managing that sort of thing when they can just use a webmail account to go around it.
I can see the argument for blocking webmail domains in order to get cleaner data, but even that seems risky as smaller companies sometimes leverage webmail accounts to do their business.
Personally, I prefer a "meet people where they live" approach. Let them use whatever e-mail address they prefer so they can manage their e-mails and calendar how they like. (Rather than creating a flow with obstructions that might turn them off the idea of registering for an event or that will make the process harder for them than it needs to be.)