You are unable change the user details such as E-mail either in bulk and/or for an individual user in MDM Cloud.
The following causes may prevent you from modifying the user details:
User name already exists
Ensure the specified user name is not already associated to another account in the org.
Domain already modified
Ensure the AD-based domain has not been modified before. If modified previously, MDM doesn't permit another domain change.
Account not yet confirmed
Ensure the verification process on creating an account in Zoho Accounts has been completed. The verification process usually involves verifying the e-mail specified during account creation.
E-mail specified is invalid
Ensure the e-mail address specified adheres to the format user@domain.
Account not yet modified
Ensure the new e-mail address specified, has also been included as the primary e-mail address in Zoho Accounts. Modify the e-mail address as explained here.
E-mail address mismatch
Ensure the e-mail specified and the e-mail address in Zoho accounts is the same.
No account associated
Ensure the e-mail address has a valid Zoho account.
E-mail wrongly associated
Ensure no new account has been created for the modified e-mail address as this prevents proper mapping from the existing e-mail address to the modified address. Instead the exisiting e-mail address is to be modified in the same Zoho account, which was previously created.