In short, it doesn't work.

external account in zimbra,

IMAP,

about 500m in size, 'get external mail' - - doesn't

fails with Fetch Failed.

Seems there is a limit to how large an external account zimbra will
accept.

What is this limit?

There is an open bug report on this, it's been open and assigned for
some time, and no traffic at all.

This is a stop for me.