Members can edit their Joomla profile, change their e-mail address to a new e-mail, and “unlink” themselves from the Member record with the old e-mail address.
That's why we created the plugin Syncemail2mms. It triggers when the Joomla e-mail address is updated, and updates the MMS e-mail address of the member.
When an account is created in Joomla, the e-mail address has to be verified using a link in the confirmation e-mail. When updating the e-mail address using the profile, this verification isn't necessary. So we created a check if the new e-mail address is already in use in MMS. If it is, the e-mail update in the Joomla profile and MMS is refused, because users could take control of other person's data that way. Also, updating an old email in MMS to an already existing new e-mail address in MMS is refused, to preserve the uniquenes of e-mail addresses.
This plugin comes automatically during the installation of mms.