Attachment problem - can't send e-mails via SMTP with attachments (Thunderbird)

If our PC users try to send a e-mail with an attachment via Thunderbird, most of the time they got an error "APPEND failed". The Thunderbird status-window stops the transfer sometimes after 5% up to 95%. But after some tries (about thee to five times) the mail is mostly send.

We only have this problem with our PC/Thunderbird clients - e.g. Mac/Mail.app never have this problem. We are using a Zimbra 4.5.3 Network server with SMTP via TLS and about 30 Clients (PC, Linux, Mac).

What is wrong? Where should I start to locate the problem? Any ideas? Who can help?

Thanks in advance, Jannik