Share Mailbox, change sender e-mail adress


I have the following 2 problems to solve on a UCS.

  1. an external email address should be queried via fetchmail and the mails should be distributed to multiple users. (Works so far with forwarding, but optimal would be to forward the mails to a mailbox and make this mailbox accessible to multiple users. But I don’t know if this is possible).
  2. sending from the UCS is for technical reasons unfortunately only possible via a relay-host. There is a relay-host too, but it refuses the mails because of a wrong RCTP TO command.
    Is there a way to change the sender address depending on the sending user?

I use UCS4, postfix, fetchmail and Kopano (incl. Web Client).

I would be happy if you could give me some suggestions.