[Dovecot] rule of thumb for indexing overhead

[WJCarpenter]

> Also, are there any drawbacks of using exim to do the local delivery?

I'm very interested in the answer to this question, too.  So far I have found (through reading, not trying things yet) that
Dovecot's quota handling is more flexible than Exim's (exim is pretty much limited to FS quotas, I think, which is no good for
virtual users).  Dovecot's Sieve implementation has more features than Exim's.  Both of these happen to matter to me (and led me
to Dovecot in the first place.)

An upside for Exim doing the delivery is that you theoretically can arrange for some additional rejections to happen at SMTP
time.  Pragmatically, those additional rejections are typically not done at SMTP time anyhow (e.g., an explicit fail from a user
Exim filter).