That is not a SOGo or any IMAP clients problem.
They can do nothing about that.

That is so because of IMAP servers definition of mailbox seperator characters.
The only way in dealing with that is to instruct users.

Hi, thanks, sounds reasonable and is something I was thinking of to.

But may be there is any possibility for SOGo to "catch" such separators and show a wring pop up window?

TB for example prevents creating folders like "foo.bar"

Something like "Invalid character (dot or slash) in your folder name!" would certainly be possible.
But I assume, that will not happen before SOGo 3.0, as this is a webinterface change.

Notice that accepting the mailbox separator character allows to move a mailbox by renaming it (ex: if mailboxes "foo" and "bar" exist, rename "bar" to "foo/bar" and it will become a subfolder of "foo").

We should probably refuse to create or rename a mailbox that contains the separator character and add drag'n'drop of mailboxes.

I "up" this functionnality. As my users only use the webmail v3 now (dovecot Maildir backend), this problem has already cause some disagreaments in mailbox trees when renaming a root folder as "1.First folder" !! Needing admin action to correct it directly on server...
Maybe can it be possible in a first time in sogo.conf to add a variable like a table of prohibited characters ? Even if there is no popup. I prefer explain to a user why his folder refused to be created due to invalid char., instead of repairing a mailbox for an hour (or more) !

Please ?