Forwarded EMail rejected by Strato since short

since some time, Strato rejects mails, coming from itself (, which are forwarded to an external mail address. Return message is:

Letzter Fehler: 550 5.7.1
Erklärung: Unerlaubte Domain ‘’ im Envelope-From (B-EFROM-STRATO)
English: Unauthorized Domain ‘’ in Envelope-From (B-EFROM-STRATO)

I do not know how to face this - so let me explain my setup:

  • I have UCS installed with Kopano
  • The User has in UCS Admin Console User Advanced Settings Category unden EMail forwarding an external address configured to forward to.
  • That works for internal mail, aswell as mail received from external sources, apart from strato’s own domains as sender addresses. It worked until maybe June / July, but stopped since.
  • I use fetchmail to fetch my mailboxes (which are hosted by Strato), and fetchmail delivers them to the UCS postfix
  • postfix delivers either locally or via Strato smtp relay
  • Its when the forwarded mail is received by the relay that I get a mail with the error message back.

I see two options:

  1. prevent all forwarding of mails from strato to the external account (not prefered)
  2. ensure that another Envelop-From is used when delivered to the relay, maybe onyl for mails from strato.

Neither one I know how to configure. For the second I’m even not sure which Envelop-From is used. The forwarded message contains an X-Envelope-From: - is it this one? Or how is Envelop-From setup in a forwarding case?

Anyone has some ideas how to tackle this?