It sounds like a "stuck" (large) email to me, it sometimes happens.
If the previous advice doesn't work you could try this which I have used successfully in the past ....
There is a piece of software you can download free from the internet called POP3 Scan Mail Box, this allows you to look at and/or delete any mail that is sitting on your mail server without actually downloading it.
Once the software is downloaded you will have to configure it with your account details and password.
It is available to download free from here:
http://www.kempston.demon.co.uk/smb/index.htmlThings that will help you when configuring it:
The PO3 server for Freeserve is
POP3.freeserve.netYour account name will be the bit in bold of your own Freeserve address eg. xxx@
YYYY.FREESERVE.CO.UK Your password is the one you normally use to log into Freeserve.
If you manage to configure this ok for your Freeserve account you will then be able to delete files on your mail server without downloading them.
Good luck !