It seems that several mail clients and MTAs have problems with quoted-printable encoding (see also FS#1755
). Eg. the iphone Mail clients barfs on qp encoded mail subjects.
One solution would be to switch to base64 encoding which seems to be better supported (this has to be verified).
The disadvantage is that mails will no longer be easily readable in raw message format and that base64 will have more space overhead for messages consisting of mainly ASCII text.