Unable to change the email address used for a Marketo user's login
All of a user's activity is based on that specific login email address. If somehow there was a way to change the login, all that history would be gone or mixed up with multiple login names and hence Marketo doesn’t allow to change the login email address.
You can however, create a new user by inviting new user in the Admin area under Users & Roles with that new email address and a new login will be created that can now be used with that email address. So you can now use that new login and delete the old if you want.
This login email address is different from the communication address. It is just like a user name for login purposes. All the communication/notification emails will be sent to the email address mentioned in the email column of your account and this should be a valid email address. You can check this under Admin > My Account > Email.
If your instance does not have the feature allowing the login email address to be different than the email address used for communication, please contact Marketo Support to have it enabled.