Attachments problem - download again when forwarding and replying

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Hi All

I'm using Zimbra 3.1 on Fedora 4 as IMAP server, and using Thunderbird as client. Whenever an incoming email has an attachment, Thunderbird takes a long time to download (it was faster previously when we used imap-uw, sendmail). But the slowness is bearable. But when I click Reply in Thunderbird, it seems that the downloading of the attachment starts all over again. Same thing happens when I forward the email.

Has anybody else experienced the same problem?
Any resolution?

Thank you.

Gui