Weird glitches for N messages sent to list simultaneously

I've been noticing this for quite awhile: if I commit to all active
branches, that causes N (currently 6) messages to be sent to
pgsql-committers at once. Very often, only N-1 or N-2 of those messages
come back promptly; the rest are delayed for periods of minutes.
(It seems like they will come through if someone else posts a message
to some PG list, though I wouldn't want to swear there is a connection.
Otherwise it can take quite a few minutes.)

I noticed this just now with respect to f64340e7436d0f84 et al, and
thought to look at the mail archives page for pgsql-committers, where
I see just four messages, same as what came back to me. So it's not
just me; there is something odd going on in message submission.
Anybody care to speculate what?

[ Waits a bit before posting ... the missing two messages did show up,
8 minutes after the first. ]

regards, tom lane