G'day einhverfr,

Xcode is Mac OS X software. The problem is at the tester's end. There's nothing further for you to do regarding this report.

If you look at "CPAN Testers Reports: Reports for Mail-RoundTrip", you'll see (in the second table):

Grade:        UNKNOWN
Perl version: 5.18.0
OS name:      Mac OS X
OS version:   12.4.0
Architecture: darwin-thread-multi-2level
[download]

If you look at the report you linked to, you'll see much the same information in the perl -V output (under "Platform:"):

  Platform:
    osname=darwin, osvers=12.4.0, archname=darwin-thread-multi-2level
    uname='darwin trushel-macbookpro.local 12.4.0 darwin kernel versio
+n 12.4.0: ...
[download]

Knowledge of Mac OS X and Xcode is not something you could be reasonably expected to have.

I have a very similar system:

$ perl -v
This is perl 5, version 18, subversion 1 (v5.18.1) built for darwin-th
+read-multi-2level

$ uname -a
Darwin ganymede 11.4.2 Darwin Kernel Version 11.4.2: Thu Aug 23 16:25:
+48 PDT 2012; root:xnu-1699.32.7~1/RELEASE_X86_64 x86_64
[download]

I've just installed Mail::RoundTrip (v0.02) without incident. I'm also a CPAN Tester so, when "CPAN Testers Reports: Reports for Mail-RoundTrip" is updated, you should see a PASS for 5.18.1; Mac OS X; darwin-thread-multi-2level. [While previewing and checking links, I see that page is now in an update queue: it may be updated by the time you read this.]

It's good that you "... try very hard to address all failure and unknown responses ...", but UNKNOWNs like this are beyond your control.