This behavior with Thunderbird 1.5 ( tested on Linux and Windows ) was caused by having the .forward file
| "/usr/local/libexec/dovecot/deliver"
in addition to postfix's
mailbox_command = /usr/local/libexec/dovecot/deliver
Once the .forward file was removed, all the newly forwarded files appeared normal. The problem existed only when forwarding as attachment, not inline.
Why ? I don't know.
Just switched from uw to dovecot ( latest beta3 + lda ) on production system and everything seems fine except... There's some strange problem with forwarding messages from thundebird, the body of the forwarded message is always completely blank - 0 byte size. Forwarding from squrrel, horde, eudora and outlook works fine. Client setup on thunderbird installs hasn't changed a bit. I'm still investigating why this is happening.
Has anyone run into this ?