[Exim] A few fairly basic questions ?

Hi All

Fairly new to all this so hope explanations can be kept reasonably simple.

We are building a linux mail server to send purely opt-in emails to our
existing customer base. This will be a monthly personalised newsletter. NB
Under no circumstance will we ever SPAM our, or any other list. Our
customers (c 100,000 records) are all stored within a MySQL database with
full contact and product purchase history. The sent Email is HTML format,
with personalisation achieved via PHP parameter substitution. Full
unsubscribe functionality is built into every Email.

The questions !

1. Where does Exim sit in relation to Sendmail. I assume it is a direct
replacement ?
2. What advantages does Exim hold over Sendmail ? Especially is it/can it
be substantially more performant ?
3. We are 'driving' our current Email sending via a programmatic PHP
interface to our MySQL customer Db ? This currently uses the PHP mail()
class. We are finding this very slow and cumbersome ? Any tips or advice ?

All help and advice will be gratefully received.

Steven Teague
s.teague@???