E-Mail setup question about a forwarding SMTP server

[esc]

I have a customer who runs an account with an external provider. We will probably shift this account to our VPS in a while. Just now I’ve set up only a local mailserver to forward email to this external customer’s account. I did this by creating a local account via WHM, created an email forwarder and pointed the external MX record to this account’s IP – seems to work so far, as my test emails are delivered correctly.

But now I just realized there’s some frozen email in the local mail-queue. It’s some spam anyway, but I wonder why it was not delivered. The target email address is valid.

The error message reads (only partly shown, addresses replaced):

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

  valid.customer@address.com
    (ultimately generated from valid.customer@local-address.com)
    SMTP error from remote mail server after RCPT TO:< valid.customer@address.com >:
    host external-customer-server.com [123.45.67.89]: 554 <original.sender@cs.com>:
    Sender address rejected: Access denied

I do not understand this message. Was the original.sender@ cs.com rejected, or was it our local relay? As this local account is setup in preparation of a future shift of the original account to our VPS it was created with the same domain-name as the original account. But as the local nameservers will at no time be queried this should be no problem, unless the remote receiving SMTP server does a reverse lookup for our relay.

Should I bother or might this have happened because the original sender was blacklisted?

Erich