Based on the information you have provided I can't answer, but:
If it's a POP3 account and your computer is on it has downloaded the message and deleted it from the server before you go to view it on your iPhone but after the iPhone has retrieved the message headers.
If it's IMAP or Exchange you have filters on your computer that have moved it to another folder before you have viewed it on your iPhone, but after the headers were downloaded.
There are other possibilities, but I'd need to know more about what kind of account it is, how many devices access your mail server, and how they are configured.
I have 3 computers accessing my pop3 acct, and all except one leave
the mail on the server. They all work fine and download as expected
but my iphone 3gs sometimes gives the "This message has not been downloaded from the server" message.
Problem had disappeared with ios5, but has recently come back. It seems to be random for
different messages now. Lots of posts and threads on this
since 2008, but no apparent solution. ????
Although it is possible to access a POP account with more than one email client, a POP account is designed to be accessed with a single email client only. One of the limitations with a POP account is only one email client at a time can be communicating with and/or downloading new messages available at the incoming mail server. When more than one email client attempts to do the same, the 2nd in line will be temporarily locked out from connecting with the incoming mail server since only one email client at a time can do so. An error message such as password is incorrect or this message has not been downloaded from the server will appear.
I know this is an old thread but I will reply anyway.
I have the same problem with some messages from my work email server, it is an Exchange server.
What usually happens for me is:
- The iphone Mail client just get stuck in "downloading email"
- After a while I get the "this message has not been downloaded from the server" message.
How to fix it (workaround because the problem can come back with other messages):
- Totally close your Mail app (press HOME button, then press it again 2 times, tab and hold over the Mail icon on the taskbar, press the X that appears on the Mail icon after a while).
- Turn off WiFi under SETTINGS
- Restart your iphone (maybe not necessary but I do it anyway)
- Open Mail accessing it on 3G
- Go to the problematic message. It should download ok.
- Once it is on your phone it should stay there.
Don't know if this will work on POP3 accounts but it's worth a try.