My understanding from the manuals is that -r sets the return path and -f sets the sender's email address, however I'm finding that they both set both, and whichever one comes latest takes the priority.
Since this binary is actually provided by Exim4, should I assume all bets are off regarding the behavior of classic sendmail?
This is what I see from the exim doc,
-r -> This is a documented (for Sendmail) obsolete alternative name for -f.
So, both the arguments are same :)
Yes, you can't compare the behaviour of the two for anything other than the fairly straightforward cases. Either the sendmail behaviour was misdocumented (so it was implemented wrong in Exim), not tested correctly (someone made an incorrect assumption when they implemented it in Exim), or it used to work that way in Sendmail (at some ill-defined point in the past) and it's changed in newer versions of sendmail.
On the other hand, if the Exim sendmail man page says something, and the sendmail compatibility shim provided by Exim acts differently, that's a bug in Exim, and should be reported and dealt with accordingly.
