There is growing evidence that an increasing number of visitors viewing web sites and emails are now using mobile devices.
A quick view of Google Analytics for some clients indicates that over 10% are currently using mobile devices to view their web sites. The suggestion is that the percentage now viewing emails may be even higher. These are not statistically validated research findings, merely observations based on what we actually see happening. The problem is that the web site does not always work too well on mobile devices.
Emails may have communication problems other than how an html email is displayed on an iPhone or iPad, but also in how the message is handled by the recipient. For a long time we have tended to use email not just as a quick message system, but as a tool to deliver a more complex document as an attachment, with a brief covering note in the body of the email. There is a suspicion that the frequent lack of response suggests that the documents are not being opened or even being received at all. So where possible the document content now becomes transmitted within the body of the email. By of necessity shortening the document this may actually improve the focus of the report. But then another problem arises in that many recipients simply forward it to someone else within or even outside the organisation. So don't send anything confidential is the thought.
May be there is a psychological issue at play here in so far as emails received on a phone are thought of as similar to text messages, some ephemeral, even trivial communication. So before sending out the next email, check how it will display and think about how it might be received. And finally, why don't people change that "sent from my iPhone" footer.