eM Client vs Mailspring

Such as sorting through categories, etc.

It is free for 2 accounts and as long as you don't use it commercially. Paid version comes with support and can handle more than 2 accounts.

They've been developing and improving it consistently for years.

eM Client seamlessly integrated the Quick Text extension that was available on Thunderbird. This makes it easy to quickly auto-insert dynamic text, based on many variables, saving time and the need to tediously repeat manual text replies.

eM Client includes integrated chat, so you don't need another application for chatting with your friends. All common chat services supported (Google chat, Jabber and others).

eM Client sends straight .pdf files for fax messages flawlessly.

It saves space, unlike Thunderbird.

But only because it has no alternative.

It provides (almost) complete Gmail functionality (calendar, tasks, chat, contacts), while offering advantages of an application: offline use, more e-mail providers in one place, very good design, support, etc.

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.