When I attempt to add a portal user with an email address of the form <base>+<discriminator>@<domain>, I receive a message that there was an error adding the user. If I specify an email address without the '+' character in the local part of the address, the operation is successful. There is no information in the logs to provide details but I suspect Zoho has made the common error of making assumptions about what is valid in the local part of the address.