I have discovered one small bug on mailuser page
when you log in on https://ispcdomain.tld/mailuser/
In "User & Email" in Field "Name:" you can input name with any diacritics
as characters ěščřžýáíé
which is actually not allowed
as you can try do this in ISPConfig https://ispcdomain.tld/
you'll get message:
Field: Real Name
Only the following signs are allowed: a-z A-Z 0-9 - _ .
These characters are saved an you can see them later in ISPConfig
(but again - cannot save them in ISPC and must correct it)
There is no problem with functionality as in /etc/passwd the Real name is simply not saved
IT should be good add allowed characters checking also at mailuser page..