We used two small Sun E250s to send upwards of a million emails a day using qmail and a tweaked version of the Solaris TCP settings to close connections as quickly as possible. These two servers also handled the bounces as well as the usual spam attempts. If I was doing something similar again I'd still used the Solaris/qmail combo as it was very reliable.
Our biggest problem was with people marking us as spammers even though they had to sign up to groups to use the service but that's a constant headache in this game. We also had some shell scripts that culled SMTP connections (either incoming or outgoing) that had been help open too long as that helped us deal with DOS attacks.
The site also throttled SMTP traffic to give HTTP traffic priority if required as that's OK for SMTP with its store-n-forward architecture but not for HTTP