chown/chgrp without chmod -s? lost postfix mail?

[Brian May <bam@debian.org>]

Hello,

yesterday I changed some of my UID and GID assignments. Including
GID=postdrop, used by postfix.

So I did:

chgrp postdrop /usr/sbin/postdrop /var/spool/postfix/postdrop

and restarted postfix. Fortunately, I ran a test mail, to ensure that
mail was still working - it wasn't.

The reason? When I typed in the above command, it disabled the setgid
flag from postdrop. Argghh!

I turned it back on, and restarted postfix. Only, the same copy
postdrop was still running, so I killed it - this in turned killed the
mail which was in transit, which was silently dropped (at least I
haven't received it yet, and it has been over 12 hours; mailq reports
an empty mail queue).

So, for future reference, is it possible to:

a) change the GID of an executable file without resetting the 
S bits?

b) restart postdrop without the risk of loosing any mail that is being
delivered?

Fortunately, I only lost the test mail I sent myself (I think!), but I
am surprised that postfix didn't queue the message and try to re-send
it latter (is this a bug?).
-- 
Brian May <bam@debian.org>