Web based email client?

Question

Mail client

Actually, there is a small organization. There are several servers on different mail servers. There is a strict restriction on access to the Internet.

We need an email client with a web interface that can work with several mail servers in the spirit of the usual The Bat! or Microsoft Outlook.

I know about Roundcube Mail, but it has a problem that for each mailbox it requires you to log in with a server selection. Those. inside, like, for example, the same the bat!, there is no tree of boxes.

And, yes, I know that there are mail collectors, but this is not suitable for us. In this context, we will quickly exceed the quota for the size of the box (many attachments) plus, in which case, there is a chance to lose everything. And so I will create another reserve point in the form of a local database (therefore, there are no special requirements for imap). For any mail service to work properly on the web, it needs to add quite a few hosts to the white list, and I'm not sure that it will be possible to get all this through security (there's just a traffic jam ...), and if a new one is added, it's a big and long epic with the addition of it.

And, well, I wanted free)))

Thank you.