st vs Mailspring

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.

Mailspring has great integration with gmail features and tags.

The UI is very well designed and neat.

Activity tracking is built into Mailspring so you get notified as soon as contacts read your messages and can follow up appropriately.

Mailspring uses a C++ sync engine designed to be as efficient as possible, so you can leave the app running and not see your laptop battery life drain away.

Using a single inbox for all of your email accounts helps you get more done in less time. Mailspring supports every major mail provider—Gmail, iCloud, Office 365, Outlook.com, Yahoo!, and IMAP/SMTP—so you have a single, streamlined command center for all your messages.

You can swipe to archive / snooze messages and specify when you'd like them to resurface in your inbox.

Only in the paid version though.