Mailu is already set up to disallow non-existing identities at the SMTP level. People can already set up identities on external email clients. This should allow users to set up multiple identities within Roundcube
Roundcube can be parametrized so it can take a different hostname than
'front' or 'imap' to connect to the mail servers through environment
variables. Unfortunately, this was not correct and in PHP a `||`
operator always returns a boolean. It did not work as expected.
Instead use the ternary operator `:?` that works in all cases.
hoellen