Users report problems when viewing messages sent from Mac Mail software, particularly when the message includes file attachments.
If your sender from Mac Mail attaches the file before they enter the message text, you sometimes won't be able to see the attachment in Web Mail. It seems to only fail on plain text messages, not on HTML text. If they reverse the order and enter their text before they add the file attachment, you will get the attachment just fine. This problem is unique to messages sent from Mac Mail.
There is a related problem where if the Mac Mail sender attaches a file, and then enters HTML text afterward, then sometimes you will get the attachment but won't see the text. Reversing the order when sending fixes it, and it also doesn't seem to occur with plain text, only formatted text. This problem is NOT unique to viewing messages in Web Mail, but happens in most (or all) non-Mac email programs when viewing these messages from Mac Mail, and this one is definitely a bug in Mac Mail. It will happen when viewing the messages in Microsoft Outlook or Outlook Express, and most other email programs.
Both of these are due to the unique non-standard way that Mac Mail constructs multi-part messages, and these don't happen with email from other sources. We will try to get it fixed, at least the attachment problem. The text problem mentioned second is in Mac Mail and we may not be able to do a workaround for it. Apple should fix it.
In the meantime, ask senders using Mac Mail to enter their message text first before attaching files instead of the other way around, and there shouldn't be any problems.