Postbox vs GyazMail

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.

Goichi Hirakawa, the main developer of GyazMail, has been committed to his project since 2003 and delivers updates regularly.

The user can define complex mail sorting rules for incoming and outgoing mail.

As of v1.6

GyazMail is a native macOS application, based on the Cocoa API.

the mailbox view is very comfortable

GyazMail is a Japanese product, so it has very good support for Asian languages.

GyazMail is available for all versions of Mac OS X / macOS starting with version 10.1

In GyazMail you can set a maximum number of characters per line. The program will wrap the lines using hard wrapping on sending. This feature is very useful for newsgroups that require a hard break after 72 characters.

Uses Oniguruma regular expression library, which is a very elaborate regular expression engine that supports a big variety of character encodings. This makes very much sense with e-mail, as in an e-mail theoretically any character encoding could be used. Because it is free software (BSD), written in C, very elaborate, stable and still actively maintained it is also used in Atom, Take Command Console, Tera Term, TextMate, Sublime Text, SubEthaEdit, and jq.