I have about 12.5k messages in my inbox at FastMail, and another 26k in separate folder. Both inboxes load instantly, and the infinite scrolling is seamless. Ju

I have about 12.5k messages in my inbox at FastMail, and another 26k in separate folder. Both inboxes load instantly, and the infinite scrolling is seamless. Jumping back-and-forth between message and list view is equally quick. While they currently lack features like a calendar, they do have a lot of powerful tools -- I have several domains and email accounts conditionally filing mail into several folders, with custom personalities/signatures when replying from each folder. Very happy with it, and the devs take bugs seriously whenever I message them. (To be fair, it's usually just UI edge cases.) Edit: Just to quickly elaborate on the infinite scrolling: FastMail takes the total number of messages in a given folder and uses that to calculate the size of the viewport. When you scroll to a given position, it fires off a small AJAX request for just the messages that are in view. Since each message is a fixed height, it gives them a lot of flexibility for calculating what to show. Also, FWIW, they do support message threading.