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> Any clues would be greatly appreciated.
>
Off the top of my head and untested - how about getting rid of the rewrite rule in the rewrite section, have a router with unseen set that checks for this particular pattern in your address, delivers it to the address you want these mails copied to, then have another router matching on that pattern that uses a particular transport that has headers_rewrite set up (see
http://www.exim.org/exim-html-current/doc/html/spec_html/ch-generic_options_for_transports.html for details) and delivers the mail to gmail ?
