Inbox actions are the buttons you see to right of previews in your inbox, for things like confirming subscriptions, tracking parcels, responding to calendar invites and so on. Postmark have written a helpful guide to how to implement them.
Note they only work if you are sending a reasonably high volume of mails.
A really useful tool Google launched last summer – it’ll give you spam rates, delivery errors, IP and (crucially) domain reputation by IP address for all messages passing through Gmail.
MailChimp have just announced that Mandrill, their transaction email service, is being discontinued as a standalone product over the next couple of months. In future all accounts will need to be tied to a MailChimp paid plan.
There are various options should you decide to take the opportunity to migrate elsewhere (MailChimp themselves mention Amazon SES) but I’d like to put in a good word for Postmarkapp; they’ve been around for ages (the company is Wildbit who also make Beanstalk), the site is nicely presented but simple and easy to use, they’re very open about how they work and they’ve launched an excellent Templates featured which I tried a few months ago.
Update 27 Feb: