[JamesHenderson]

Originally Posted by TheJapanese

Thx for your feedback.

I made a standard rule like:
Any "To"/"CC"/"BCC" email-address is alias@domain.de > Move the message to subfolder 'aliasfolder".

The rule is working.

But like I said, if I get a mail from someone who is writing to my main address (mail@domain.de) AND to alias@domain.de then the mail only appears in the subfolder aliasfolder (and is not in my INBOX).

You need two rules:

Any "To"/"CC"/"BCC" email-address is alias@domain.de and no "To"/"CC"/"BCC" email-address is main@domain.de> move the message to subfolder 'aliasfolder".

Any "To"/"CC"/"BCC" email-address is alias@domain.de and any "To"/"CC"/"BCC" email-address is main@domain.de> copy the message to subfolder 'aliasfolder".

...something like this?

if address :is ["To","Cc","Bcc"] "alias@domain.de" {
	if address :is ["To","Cc","Bcc"] "main@domain.de" {
		fileinto :copy "INBOX.alias";
	}
	elseif not address :is ["To","Cc","Bcc"] "main@domain.de" {
		fileinto "INBOX.alias";
	}
}

(I haven't checked it for errors)