There is definitely a problem here, though I see a different problem when I run it (AttributeError: 'Group' object has no attribute 'local_part', presumably because of the ':' not getting escaped correctly).  I believe it applies to any address header, not just Reply-To.  Unfortunately I don't have time to investigate the cause, at least right now.  An interesting first step on diagnosing it might be to produce a minimal example: start deleting special characters from inside that quoted string until you find the one (or ones) that is triggering it.