commas

I ran into a problem which seems to be that a mail list server
(mreply) tried to send mail to a list of addresses, where the list was
written as:

a, b, c

I'm guessing that mreply passed this to exim on the command line,
like:

exim a, b, c

This doesn't work, of course. But

exim a b c

works fine.

Is this common usage? Should exim strip a trailing comma?