Allerdings, auf einem dritten, frisch eingerichteten Server Ubuntu 22.04 
(Hetzner) bekomme ich:

curl -v --ipv6 smtp://mx1h1.comcast.net
*   Trying 2001:558:fd02:243f::2:25...
* Connected to mx1h1.comcast.net (2001:558:fd02:243f::2) port 25 (#0)
< 554 resimta-h1p-037530.sys.comcast.net resimta-h1p-037530.sys.comcast.net 
2a01:4f9:c011:6a5b::1 Comcast requires that all mail servers must have a PTR 
record with a valid Reverse DNS entry. Currently your mail server does not fill 
that requirement. For more information, refer to: 
https://postmaster.comcast.net/smtp-error-codes.php#554
* Got unexpected smtp-server response: 554
* Closing connection 0
curl: (8) Got unexpected smtp-server response: 554
root@ubuntu-4gb-hel1-1:~# 

Grüße
Christoph



> Am 08.09.2023 um 11:07 schrieb Christoph Kukulies <[email protected]>:
> 
> Habe jetzt ipv6 wieder eingeschaltet, bekomme aber den Fehler weiter:
> 
>  curl -v --ipv6 smtp://mx1h1.comcast.net <smtp://mx1h1.comcast.net>
> * Rebuilt URL to: smtp://mx1h1.comcast.net/ <smtp://mx1h1.comcast.net/>
> *   Trying 2001:558:fd02:243f::2...
> * TCP_NODELAY set
> * Immediate connect fail for 2001:558:fd02:243f::2: Network is unreachable
> * Closing connection 0
> curl: (7) Couldn't connect to server
> 
> Und auch auf einem anderen Server (beide bei Hetzner) bekomme ich dies:
> 
> 
> curl -v --ipv6 smtp://mx1h1.comcast.net <smtp://mx1h1.comcast.net>
> * Rebuilt URL to: smtp://mx1h1.comcast.net/ <smtp://mx1h1.comcast.net/>
> *   Trying 2001:558:fd02:243f::2...
> * TCP_NODELAY set
> * Immediate connect fail for 2001:558:fd02:243f::2: Network is unreachable
> * Closing connection 0
> curl: (7) Couldn't connect to server
> 
> 
> 
>> Am 07.09.2023 um 03:05 schrieb Alex <[email protected] 
>> <mailto:[email protected]>>:
>> 
>> curl -v --ipv6 smtp://mx1h1.comcast.net <smtp://mx1h1.comcast.net>
> Grüße
> Christoph
> 

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