[exim] Maildir file names

Exim users,
I have a few Exim servers all running latest Exim.
However, one server in particular is writing Maildir filenames different
than the rest.
For example, I'm used to seeing something like this:
1213327320.M661768P70844V0000005BI00764960_0.server.domain.com,S=2076:2,

This server is only writing these filenames:
1215326800.H536521P26074.:2,S

I doubt I'll ever get a file with the same name in either case (one in ...
too high to count chances)
But I was wondering what decides this behavior? And is something configured
wrong for the incredible difference?

Regards,

--
Brent Jones
brent@???