Cannot send message using the server iCloud (Apple Mail)

My guess is that there is some anti-spam check happening when iCloud receives the email to be relayed.

For example, let's say you receive an email with these MIME headers:

From: Company ***
To: ***

When you redirect, I assume the MIME content of the email will be sent verbatim, without any modifications.

When this happens, you authenticate as Nikolai, but the MIME header says it's from ***@company.com. That would seem spammy, and if that were possible then anyone with an iCloud account could forge an email From header (DKIM signing notwithstanding).

This is my hypothesis, because I just ran into this error and tested a workaround:

1. Drag the email to a folder (e.g. Downloads)
2. Open the email in a text editor and change the From: header to use the email address that you will be sending from.
3. Double-click the email to open it, and then do the redirect.

That seemed to work for me, which is evidence that iCloud is actually comparing the From: header in the MIME to the email addresses associated with your authenticated iCloud account.

[Edited by Moderator] 

Hi Nikolai Franke,

Without more information on the specific scenario, it's possible that you're attempting to forward an email that was sent to a Hide My Email address, which was addressed to an address created per the process described in Create and edit Hide My Email addresses on iCloud.com and already forwarded to your iCloud account. Such emails can be replied to but forwarding has the following described limitation:

"Note: You can forward your unique, random addresses to an email address associated with your Apple ID. To add or remove an address, go to the Apple ID account website."

Regards.