In terms of specifics, I like qmail. In terms of spam management, there are lots of things to do. First and foremost, have daily monitors on all the major email providers. Get on all their feedback loops. I've found that contacting them with problems is annoying, but does work if you are following all their guidelines and are persistent about it.
Current stats say that our % of users who have signed up that make it through email confirmation step is about 22%.
Around 33% or so of our signups have yahoo.com addresses, and only about 12% of those manage email confirmation.
It is similarly bad for hotmail. GMail used to be quite good, but has slowly gotten worse over time.
I'd be very curious to know if others are measuring these things, and how you are doing. Feel free to contact me directly as well.
I've worked on several business that had to send confirmation emails (via our own servers), and the figures were never anywhere near as bad.
If there is any way that your users can benefit from signing up other users (e.g. if you run an MLM site), you might want to look into whether your stats are not being falsified by a few users submitting vast numbers of email addresses into your system.
You can benefit by signing up other users, but this is not a very often used feature. It's not an MLM site, but a subscription chess service.
We track signups by ip and by browser cookies, so we're reasonable sure these are distinct people. Since we switched to the freemium model, there is little incentive for people to make duplicate accounts now, so I was hoping to see activation go up.
We do domainkeys and SPF and I have gone around the block with hotmail two times already.
We send approximately 25k emails a month, the bulk of which are sending out confirmation emails to people that signed up, and then a thank you for those that confirm. The rest are forgot password emails and invites.
http://blog.fastmail.fm/2007/12/05/sending-email-servers-bes...
In my new services, I did have issues originally, especially on Gmail. So there was an initial ramp up with spam issues on the majors. But like I said, after following all their rules and then persistently hounding their support staff to investigate, I've gotten past their filters.
There are a few things you have to get right (DNS including a PTR for your IP, SPF and now DKIM, RFC compliance in your server and configuration), but once that's done, bounce rate will be nearly as low as the quality of your list allows. Hitting the spam filters isn't all that bad either--assuming your message isn't spammy on other counts.
One other thing to be wary of: Sending to the same domain many times in rapid succession. Be sure you're not sending more than one or two messages at a time to Gmail or Yahoo or Hotmail (or any single server). So, randomize the list, and sleep for a second or two between each send, or something, to insure that there are pauses.
Oh, and at least Google's spam filters are reputation-based (and probably many others are, as well). The looser you play with the "opt in" requirement, the more likely you are to be flagged as spam by your recipients. It only takes a few to get your future mailings dumped into the spam folder. At that point your server itself is poisoned from sending to that destination for a while. So, make sure your recipients actually want what you're sending them.
Did you do anything in your installations to address that problem? If so, what was it?