I have a weird problem.

Some (not all!) customers complain, that they can not properly receive
eMails with MIME-Type: .pdf

Instead of attaching (and viewing) .pdf correctly
they report the following in the body of their eMails:

--trekuen-9610e881-3f8b-4bff-a863-76fa3c518b7b Content-Type: application/pdf Content-Disposition: attachment; filename=70755199.pdf Content-Transfer-Encoding: base64 X-UI-Content-Hash: 87308c53e05d730ce20b0b1b8d9e6d9ad3541052e8c7cf9c06b972a4012326a4 JVBERi0xLjMKMyAwIG9iago8PC9UeXBlIC9QYWdlCi9QYXJlbnQgMSAwIFIKL1Jlc291cmNlcyAy IDAgUgovQ29udGVudHMgNCAwIFI+PgplbmRvYmoKNCAwIG9iago8PC9GaWx0ZXIgL0ZsYXRlRGVj b2RlIC9MZW5ndGggMTY1Nz4+CnN0cmVhbQp4nJVXy3bbNhDd5ytm03OSUwkGQIAgvaoVSbYT23Uj J219sqEkUELNRwpSVeKfbRf+hq686IDUg9QjTexzbIu+mLmYx50hhzcvKJEKli/+BI9SIhRQ/A4V oQFwPyQ8BMV893ySwsklg34Ov7zo3cHJkIFPKIW7GAZ37onkHuES/NAjAh9P4eVlVmp7o8vfXsHd ....


(this is only the extraction!)

So instead of seeing the .MIME type correctly I see those base64-coded line.
It only happens with the provider gmx.net and web.de in Germany.
It looks as if they use a similar engine.

We use:

                        

Any ideas would be appreciated.