I don’t have a guide for this specifically but basically you can create a file inside /etc/exim4/conf.d/rewrite/00-my-rewrite-header
And use the basic rewriting of exim.
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The flags have the following meanings:
- S - Do all rewriting at SMTP time. This means that all of the following flags are done immediately as the data they apply to is coming in, not delayed until later.
- F - Rewrite the envelope from field.
- f - Rewrite the From: header field.
- r - Rewrite the Reply-To: header field.
- s - Rewrite the Sender-To: header field.
Read Chapter 31 of the Exim Specification for more detail on message rewriting.
Credit for the answer on ServerFault: email - Exim: Change sender address when sending mails out of local network - Server Fault