Csup email client

Terminal-based email clients may seem like an anachronism from the 1970s, but they have their place even now. They don’t require a browser, they can run on servers via ssh, and they are very fast to operate, being keyboard-driven and character-based.

I have been using these kinds of email clients since around 1986, when I was working at Digital Research and we got a DEC VAX running 4.2BSD Unix. I continued using terminal-based clients when I started using Linux, especially mutt.

In 2008, as I was becoming familiar with the Ruby programming language, I discovered sup, an email client written in Ruby. Sup had a number of improvements over mutt, some of which were inspired by gmail. It had a very fast search feature, it used tags instead of folders, and it displayed threads in a tree-like fashion.

I used sup until around 2017, when I discovered a fork of sup that replaced sup’s own search index system with notmuch. This appealed to me, because I had found sup to be not entirely robust about the integrity of its search index when it would crash. Occasionally I would have to rebuild the index from scratch when it got corrupted. By offloading the searching to notmuch – which ran as a separate program, was in active development, and had achieved good stability – sup could gain some of that stability.

(There is also a certain amount of irony in the existence of sup-notmuch, since notmuch itself was inspired by sup.)

I forked the notmuch-enabled sup, and called it sup-notmuch (github mirror). After a fair amount of hacking and bug-fixing, I was able to get it to be stable, and I used it for the next seven years.

Around 2023 I started moving many of my software dependencies, like email and web hosting, to a Linux VPS. Because my email was now being stored on this server, I was able to run sup-notmuch there as my email client. This allowed me to use ssh from any machine to access my email.

However, I noticed that sup-notmuch used a very large amount of memory, about 145 MB, which was a bit of a load on my server, which doesn’t have a large amount of memory. So around the end of 2019, I started rewriting sup-notmuch in Crystal, which is a compiled language similar to Ruby. I had to put the project on hold until late 2023, when I resumed work in earnest. By the middle of 2024, I had a working email client that was functionally nearly identical to sup-notmuch, but was now a single compiled binary that was much faster and smaller.

I call this program csup, short for “Crystal sup”. It’s terrible name, and unpronounceable, but I am sticking with it because it’s easy to type. I’ve been using it full-time for just over a year, and it works well enough for me that I no longer feel the need to add any more features.

The main thing that csup is missing is support for gpg-encrypted emails, but I never needed that feature and don’t expect to need it in the future. It’s also missing most of the hooks from sup-notmuch, but has enough for my use.

In csup, I was able to eliminate nearly all of the parsing of emails in sup-notmuch, by using notmuch itself for this purpose. I also added an SMTP client to csup, so that an external email sender (like msmtp) is not needed.

The source code, including full documentation, is in a Fossil repository here. I also have a github mirror here.

I don’t expect anybody else to use this client, but I do believe it’s usable, and that sup users (if there are any left) might be happy with it.