What does rejected after DATA: malformed address: xx@yy may not follow <xx@yy> : failing address in "from" header mean?

Your DATA ACL contains

verify = header_syntax

and an incoming message contained the line

From: xx@yy <xx@yy>

This is syntactically invalid. The contents of an address in a header line are either just the address, or a phrase followed by an address in angle brackets. In the latter case, the phrase > must be quoted if it contains special characters such as @. The

following are valid versions of the bad header:

From: xx@yy
From: "xx@yy" <xx@yy>

though why on earth anything generates this kind of redundant nonsense I can't think.