Hi, LegionFX!
The logic is following:
Mailbox user is not the same as Panel user.
On Subscription Control Panel you can see tabs "Mail" and "Users".
On "Mail" tab you can create mailbox and on "Users" tab you can create users to login CP.
Plesk 11.5 has additional checkbox on mailbox creation screen (by default it is enabled). It means that if you are creating email address - the CP user will be created automatically by default, so the user can send/receive mail and login to CP with same credentials.
If you need only to send/receive mail for user and do not need access to CP - you can just disable this checkbox on email address properties or during creation.
If you need only login to CP and do not need mailbox for the user - open Users tab and create it with "Use an external email address" option (where you can specify external address, such as gmail one). By default Username for such User will have external email name for login to CP, but it can be changed (during creation or later on User properties screen).
Also on User creation screen you can specify internal email address with option "Create an email address under your account". In this case mailbox will be created automatically with same credentials.
Am i right that you want some mailboxes (
[email protected],
[email protected]) handled by GApps mailserver and others (
[email protected] ...) handled by Plesk postfix mailserver when domain.tld served by Plesk?
If so, then such case requires sophisticated custom configuration of mailserver and not realized in Plesk yet.