Exim4 - Change sender address

begin rewrite
myapp@myserver.mydomain   legal.user@myserver.mydomain   SFfrs

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

There has to be a ^ in the beginning of the rule, as S excepts a regexp.

s should be “Sender:” header not “Sender-To:” (And it is a too tiny change to edit the post).