Sorry this didn't get replied to sooner.
The condition failed is defined in exim's default config file
40_exim4-config_check_data
quoting from the exim spec (aka manual):
BEGIN_QUOTE
This condition is relevant only in an ACL that is run after a message has
been received, that is, in an ACL specified by acl_smtp_data or
acl_not_smtp. It checks the syntax of all header lines that can contain
lists of addresses (Sender:, From:, Reply-To:, To:, Cc:, and Bcc:),
returning true if there are no problems. Unqualified addresses (local parts
without domains) are permitted only in locally generated messages and from
hosts that match sender_unqualified_hosts or recipient_unqualified_hosts,
as appropriate.
END_QUOTE
After reading that, I quickly see the problem: bug-gnu...@gnu.org
<bug-gnu...@gnu.org>
The first @ in that address is not valid.
