Sender Rewriting Scheme messed up when using "send copy"

[subven]

Okay I will post this in Roundcube but I'm not sure if it belongs to the app or the mailserver.

Conditions:

• Cloudron mailserver
• Mailjet as SMTP
• "send copy to" filter added in Roundcube for users mail address
• no masquerading enabled for the domain

Okay so what happens is that the user receives a mail from random@random.de to his Cloudron mail address user@myown.domain which then a copy is sent to his private.mail@web.de.
This results in a mail from Mailjet that the message was blocked due to a wrong sender address:

Hello Mr. Robot,

We are contacting you as you (or one of your team members) tried to send an email with sender address: random@random.de. But this sender address has not been validated yet on your account: 

Cloudron mail logs say:

Sender rewrite scheme seems to mess up in this case --> SRS0=1b4d=XT=random.de=random@myown.domain

Settings at Roundcube:

AFAIK I used the "forward" filter for some mailboxes before without having this issue. Is "send to" buggy? I'm not affected but one of my users ran into this issue and asked for help.

[humptydumpty]

This might be related to an issue I ran into when I tried to send a copy of my mail to my Proton email address. The problem was that Proton mail does not support SRS. Perhaps you're running into the same problem.

https://forum.cloudron.io/topic/7048/mail-forward-chaining?_=1657902017203

[subven]

@humptydumpty sounds reasonable. "Send copy to" using SRS could fail depending on SMTP features and "forward to" (not using SRS?) should work. I don't think its an real issue but I also did not find any helpful documentation regarding Cloudron and this constelation. Also bad UX in the end ^^