I have the same issue. It does not always lose the mail, just 3 times out of 4.

Since I am using IMAP I can force it to reread the messages. Some people have been able to get this to work by deleting the mailbox and recreating a new one, however this usually does not work for me (and then only if I have created a mailbox with a different name from any existing one).

I have found the following to "resolve" the issue (note requires 'X-term' be installed, and probably works best if you are talking to an IMAP server):

1. Take a copy of the /home/user/.osso/osso-email/accountrc file immediately after it is created (immediately after account creation and before downloading any mail).

2. When the issue occurs delete the /home/user/apps/email directory, and copy your pristine copy of accountrc back to it's original location (/home/user/.osso/osso-email/accountrc).

3. Try again.

Not exactly a useful solution (hopefully of use to someone who wants to debug the issue?)... and note that the file paths shown above might not be entirely accurate, as I no longer have access to my Nokia 770-- it's been wrapped up for Christmas by my wife :-(

On the other hand, by the time I am allowed to open it again, maybe this problem will have been fixed :-) !!