[Exim] Very large mailing lists (Exim vs Isocor)

Anyone here using exim with very large bulk output?

I'm doing analysis for a client that's business is mailing out to large
mailing lists. Sometimes the amount of the recipients exceeds 1 million, and
many times the body text is unique for each recipient.

I'm thinking recommending a move to Exim, but I've never used it with such a
heavy load. They are currently using IsoCor MTA and are having performance
problems. They were using sendmail and have real bad performance problems.

I took a look at zmail and my first impression is I don't like it.

Any input apprecieted.

Dave