st vs Postbox

st is built to serve as a lightweight terminal emulator. It's very light and doesn't require many resources to run, making it able to run well on older and low-end machines.

st consists of a single C file that takes seconds to recompile. This also makes it very easy to understand and customize.

st supports color escape sequences for a full 16 million 24-bit color spectrum, instead of the typical 256 colors.

There is full XFT (X Free Type interface library) and fallback font support through fontconfig in st. If your selected font is missing a certain glyph or symbol, but one of your other installed fonts has it, it will be shown.

Copyfree licensing implies that the user has the freedom to copy, use, modify, and distribute what he/she possesses.

Handles image previews (e.g. in ranger) way better than other terminal emulators.

The patches on the site are great. Scroll back, hide the mouse, etc.

Hackable and lightweight.

it is extremely responsive and fast, even on older computers.

The terminal doesn't have any bloated features nobody uses. It is very minimal and extendable.

Reminding the user that they forgot to attach the file if they mentioned one in the body of the email is one of the most useful features of any email client and so many don't include this. Postbox is pre-configured to look for certain keywords (attached, CV, spreadsheet, pdf etc) and you can specify your own keywords in addition.

Has the ability to block tracking gifs commonly used for read receipts.

All messages in one place. But you can select to show only messages from each account either.

Quick bar allows you to add a variety of content to your message. It will let you reuse URLS in your Sent folder, and autocompletes against canned responses, signatures, and topics.