According to what I have seen, the messages with "invisible" attachments I received all came from iPhones or iPads.

As peterleinchen said:

N900 does not show attachments but informs you in overview that this mail has got attached something

N9 does not show the attachments and gives you not a hint at all that there could be something attached. Only from the size of the email, you can conclude that there is an attachement.

With Jolla, it is a mixed bottle. It appears to me that most emails and attachments are now shown as they should. Only at very rare occasions, I can not download an attachment on the Jolla. Email client on myJolla never freezes.

It would be great if someone could resolve this issue! It must somehow be related to the way how iOS "packs" the attachments.

BTW: my N900 at some point started to corrupt attached .doc files so that they could not be opend by the addressee.