Admin report for changed email addresses?

I have the need to watch users changing their email address.
(Background only: We have another system to maintain members for a sports club with their email address - only users with an email address from this external system are allowed to be in the forum. Changes should be kept in sync - but not enforced hardly)

Where can I see user initiated changes of email address? It is ok for me to use pg SQL, probably preferred because it could be automated then.

Thank you

Set the other system up as an SSO provider, and only allow logins through that SSO?

Good idea, but the other system is a traditional offline member management system.

So I am looking not for a stronger coupling, but for either a notification or an on demand query to find address changes.

(The max amount of users is under 1000, changes will happen rarely. For now this is done manually, but after a while it might be partially automated)