I guess no one is using exim since this bug was introduced 6th March by the security fix and hasn't yet been rectified.
So here is my super simple fix.
I think that you may want to consider updating the exim_spec as the stubbing of the call method was hiding this bug.
Fix a bug in exim delivery method which was introduced by the commit a…
Hey there, would you like to fix the specs up to avoid regressions in the future?
Once we have this in place, if you would be so kind, I'll merge in.
Actually, merged and pushed. Please do add some coverage specs though