I can't comment directly on the mail modules but I feel it might be worth stressing that the underlying layer of mail software is also important.

I personally use qmail instead of sendmail, and although it is initially quite a chore to set up it runs itself very well once it is going.

Also don't forget that mail often hangs around on the server until the other end is ready. You will also get bounces - you should have a plan for handling these. If appropriate use a mailing list program to manage the bulk emails - yo can set yourself to be the only poster and the software will then deaal with bounces, retrying and keeping track of which emails haven't gotten through.

If you handle the bounces yourself then you may want to add a custom header to the emails so that you can keep track of who has gotten what. If this is not important you can just keep resending and only delete the email address from your records after three bounces or so.

Finally; play nice. Provide an easy way of unsubscribing, and after the user unsubscribes send an email that tells them how to resubscribe again. This is very helpful when the users are changing their addresses.