> My problem is that I'm aware a message is forwarded only at the routers level
> (the final destination e-address is taken from /etc/aliases or ~/.forward)
> and :at_start: option does not seems to work in transports
I'm not sure, but you can try:
In routers place the content you need into address_data
https://www.exim.org/exim-html-current/doc/html/spec_html/ch-generic_options_for_routers.html
In the rcpt ACL you should have "require verify = recipient" (without callout).
After that insert:
warn condition = ${if def:address_data}
add_header = :at_start:X-Forwarded-For: $address_data
This assumes single recipient, but that's usually the case if forwarded.
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