[exim] $sender_host_address is localhost - it is wrong

Hi!

I'm new to the list.
I'm running exim 4.88-5 from debian. And i have a problem.

Exim is running in original-configuration from debian, and i tried to
activate spf-check. This workes fine almost on all mails. But some got
rejected on failing spf-check. I thought, this is in case of wrong
spf-keys on the senders dns-entry.
But then i realized, the sender host ip is 127.0.0.1. So i thought, the
sender has a misconfigured mail-system.
After that i analyzed the email and found out, googles spf-check was ok.
(i receive some of my emails via gmail).

fetchmail fetches the mails and exim delivers them to dovecot without
spam-check. Only some sieve-filters in dovecot are working.

Here is the journal for one email:

Feb 13 14:47:48 aldebaran exim[12477]: 2017-02-13 14:47:48
H=(aldebaran.localdomain) [127.0.0.1]
F=<notification+ybbrsasn@???> rejected RCPT
<jakob@localhost>: SPF check failed.
Feb 13 14:47:48 aldebaran exim[12477]: 2017-02-13 14:47:48
H=(aldebaran.localdomain) [127.0.0.1]
F=<notification+ybbrsasn@???> rejected RCPT
<jakob@localhost>: SPF check failed.
Feb 13 14:47:48 aldebaran fetchmail[850]: Nachricht
username@???@pop.gmx.net:10806 von 10884 wird gelesen (21436 Bytes)
(Log-Meldung unvollständig)
Feb 13 14:47:48 aldebaran fetchmail[850]: [200B blob data]
Feb 13 14:47:48 aldebaran exim[12477]: 2017-02-13 14:47:48
H=(aldebaran.localdomain) [127.0.0.1]
F=<notification+ybbrsasn@???> rejected RCPT
<jakob@localhost>: SPF check failed.
Feb 13 14:47:48 aldebaran exim[12477]: 2017-02-13 14:47:48
H=(aldebaran.localdomain) [127.0.0.1]
F=<notification+ybbrsasn@???> rejected RCPT
<jakob@localhost>: SPF check failed.
Feb 13 14:47:48 aldebaran fetchmail[850]: kann noch nicht einmal an user
senden!
Feb 13 14:47:48 aldebaran fetchmail[850]: nicht gelöscht


You can see, the sending host is my localhost. But in the email, the
sending_host_address is a valid host-ip from the sender. I can see it in
the headers.
I tried a lot to see more about the handling. But i only found out, that
exim4 sets $sender_host_address and $sender_host_name to 127.0.0.1 and
aldebaran.localdomain.

I read, that this happens, when localhost delivers a message to exim and
not an external host... but why does this happen only on a few messages?

Regards

Jakob