In the present, email is a multi-format, multi-encapsulation medium that transmits text, binary data (thing attached images and files), and can even download content from the web after it is opened by the recipient. These complexities are mostly hidden from the user behind beautifully designed graphical interfaces that let everyone easily construct emails with tables, images, bulleted lists and all sorts of fun things that make them visually appealing.
As visually appealing as our emails have become, so too have they become technically sophisticated and complex in both content and ways that content is transmitted between the systems that allow access to it. Think about web sites for a minute, and how if you use different browsers they appear differently -- perhaps sections are in different places, images scale differently, and background colors and pictures show up or sometimes not at all. This is the same for emails -- in most cases, the emails that are being sent with these full-featured visual effects are being sent as HTML pages. Each mail client will interpret HTML emails differently, just like different web browsers do. This leads to sometimes inconsistent viewing of emails by different recipients. The more complex the email, the more variation will be seen between different email client programs.
Now, back to delivery of that email, where we have a different set of issues. The first is that there is no mechanism in email that guarantees the intended recipient received your email at all. We can use features like "Request Read Receipt"; but that requires the recipient to actually confirm they would like to confirm (if their email client supports that function at all). We have features like "Request Delivery Receipt", which confirms that our mail servers sent the message to and the recipient's mail server accepted deliver of the message. But, that again doesn't guarantee that the intended recipient received it, as there is most likely several levels of spam filtering the message still needs to go through before it ends up in either the user's Inbox or their Spam folder or discarded by one or more spam filtering services completely.
The take-away here is: if you want to guarantee that your recipients can view the message you are sending, use the simplest format (plain text) that you can. That way they can see it properly on their mobile phone, web browser mail client, or desktop. Take-away two: sending an email doesn't guarantee it is being read. Don't fire people via email, chances are they might not read that one and show up for work anyways. Same goes for any important communication that requires immediate action; it is best to request some confirmation, rather than just make the assumption that it is received and done.