What's a good open rate?

Depending on the size and age of the list anywhere between 5-15% is "good"

I think anywhere between 40-45 % is a good optin rate.
But obviously it depends on the source of traffic, list size etc.

100% would be an excellent open rate.

This is a loaded question.

If you send out 1 billion emails and have a 0.5% open rate, this is going to be really good.

If you send out 1000 emails and have a 1% open rate, this is not going to be very good.

It depends on what you are looking to accomplish.

If you have a double opt in list, 25% or so would be considered "good"

As maximum possible.. but minimum it should be 10%

Well, I guess I have to be happy from my servers and users then. We got 15-20% open rate of 20k-50k email campaigns.

Open rates are very much dependent on the business you are in! Obviously if you are a musician sending funny backstage videos, free songs and generally entertaining stuff you'll have a much easier game getting big open rates than say an accountant company sending out updates on tax laws or other musty stuff like that. Take away is, compare your open rates to those of people in the same or a similar niche. All these numbers above are completely without value to you, me or anyone if I don't know what these guys are writing their newsletter about or how they built their list. This is like comparing the price of Chinese plastic beads and a signed copy of a Pink Floyd picture vinyl.

And even inside one niche open rates will vary greatly depending on the strategy and kind of relationship you built with your audience.

The real question is not, what open rates should I get (because you should always test and try to get better) but what can I do about open rates? (And there I am not even sure if Open Rate is the right value to measure success, I usually only look at click rates)

Like you already said, look at the things you can do something about
a.) work on your subject lines (this will also help with not getting into spam, in gmail for instance)
b.) try different autoresponders - in my experience aweber, though it is a lot worse than it was - is still way better in that regard than Get Response (I suspect that 30% to 60% of my welcome emails to new subscribers in GR got into spam at once)
c.) if this is in accord with your line of business, try to establish more rapport and trust with your subscribers (by your remark about mailchimp I would guess it is not, but I still add this)
d.) ask your subscribers to whitelist your email address (also not a strategy that will work in MMO, IM niche, to state an example, but still worth a try and costs nothing)
e.) use GR (or mailchimp, benchmark, aweber etc) to collect subs but use something like amazon SES to send out emails

and probably a bunch more solutions

Hey,

With what I have been working on I could say the Open rates are pretty good considering that you are not looking at the quality of your lists.

100% delivery doesnt essentially mean it has hit the inbox, assuming that your lists contain generic ISP emails. It could be either in promotions or the social tab for generic mail isp . Do check how many emails you hit per hour on ISP as they could burn your account fast!!

There are 3 measurements of email campaign: unique open rate, bounce rate and click-through rate. The open rate is calculated by dividing the number of unique opens by the amount of email sent, minus the number of bounces. I think a good open rate is 20-25%.