German characters cause loss of line breaks in some emails.

[bazil2]

(Elementary user)

I'm trying to find out why our incoming mail server causes the loss of new line breaks when mails contain German characters like ß, ö, ä, or ü

Consider the following body text:
---------------

Line 1
Strasse
Line 3

Line 4
Aepfel
Line 6
---------------
A mail with the above body text is received correctly and has the following (abridged) header code:

Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: quoted-printable

Now consider the following body text:
---------------

Line 1
Straße
Line 3

Line 4
Äpfel
Line 6
---------------

This is received in the mail like this i.e. all in one line:


---------------
Line 1 Straße Line 3 Line 4 Äpfel Line 6
---------------
It has the following header:

Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: base64


This doesn't happen on all incoming mails only those from a specific sender.

Any ideas?