In fact, I'm using IPv4 for sending only on my mail server as it improves delivery. Should have mentioned that.
I personally disencourage people to disable IPv6 for anything, but for gmail this is a exception. But maybe we can add a setting to the postfix settings to prefer IPv4?
normal delivered email Code: ARC-Authentication-Results: i=1; mx.google.com; dkim=pass header.i=@xxxxxxx header.s=default header.b=vMWcdUxy; spf=pass (google.com: domain of info@xxxxxx designates xxxxxxx as permitted sender) smtp.mailfrom=info@xxxxx; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=xxxxx Received-SPF: pass (google.com: domain of info@xxxxxx designates xxxxxx as permitted sender) client-ip=xxxxx; Authentication-Results: mx.google.com; dkim=pass header.i=@xxxxxx header.s=default header.b=vMWcdUxy; spf=pass (google.com: domain of info@xxxxx designates xxxxx as permitted sender) smtp.mailfrom=info@xxxxx; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=xxxxxx spam folder delivered email Code: ARC-Authentication-Results: i=1; mx.google.com; dkim=pass header.i=@xxxxxx header.s=default header.b=WWtU5SJW; spf=pass (google.com: domain of info@xxxxxx designates xxxxx as permitted sender) smtp.mailfrom=info@xxxxxx; dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=xxxxxx Received-SPF: pass (google.com: domain of info@xxxxxx designates xxxxxx as permitted sender) client-ip=xxxxxx; Authentication-Results: mx.google.com; dkim=pass header.i=@xxxxx header.s=default header.b=WWtU5SJW; spf=pass (google.com: domain of info@xxxxxxx designates xxxxx as permitted sender) smtp.mailfrom=info@xxxxxx; dmarc=pass (p=REJECT sp=REJECT dis=NONE) header.from=xxxxxxx I can not see any big issues why one mail is delivered to spam and one not.
Spam Folder Email: Normal delivered email: Now i have tried a different gmail adress and there the first email was sent to spam folder the second email was delivered normal. Now i´m completly confused.
have you tried adding the email address that your sending from as a contact in the gmail account that you're sending to? it's something i've seen suggested as a solution by google themselves a number of times. i know it's not really a practical solution if lots of different source/destination addresses are involved. but maybe run a few tests from a couple of source addresses to a couple of gmail accounts... see if it makes any difference.
i'm not claiming it's a solution, i even said in the post it's not a practical solution. but it could be tested, and possibly show that there's absolutely nothing wrong with what's being sent, and that the entire problem could be strange bahaviour/decision making on gmail's side, which there might not even be a solution to. who knows, maybe it won't even make any difference, but a couple of minutes to test it ain't going to hurt, and could save you hours endlessly searching for a problem in your mailserver/emails that might not even exist, you could identify that the problem is on their side, at worst, you've wasted 5 minutes.
The content of the email matters a lot. Emails with a text like test or test2 are delivered to the spam box by most providers, so that's not suitable as a test in any way. If you want to test, use a real unique text which is not just 1-2 words. And if you sent several of these test emails, it is even more likely that further mails end up in spam folder.