When comparing Mailplane 3 vs GyazMail, the Slant community recommends GyazMail for most people. In the question“What are the best e-mail clients for macOS?” GyazMail is ranked 6th while Mailplane 3 is ranked 18th. The most important reason people chose GyazMail is:
Goichi Hirakawa, the main developer of GyazMail, has been committed to his project since 2003 and delivers updates regularly.
Specs
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Pros
Pro Can work with mutiple Gmail accounts simultaneously
The tabbed interface included in Mailplane 3 lets users work with as many Gmail accounts simultaneously as they want.
Pro Fast and light
Pro Doubles as a calendar
Mailplane 3 combines Google Calendar and Gmail within a single app.
Pro Easy to use
Mailplane 3 looks just like Gmail's web UI and even imports Gmail shortcuts. If Gmail shortcuts aren't ideal for the user, those of Mac can be used instead.
Pro Supports many plugins
Functionality and features can be extended by using plugins.
Pro Long term support and development
Goichi Hirakawa, the main developer of GyazMail, has been committed to his project since 2003 and delivers updates regularly.
Pro Mail sorting rules
The user can define complex mail sorting rules for incoming and outgoing mail.
Pro 64-bit support
As of v1.6
Pro Native Cocoa API based
GyazMail is a native macOS application, based on the Cocoa API.
Pro customizable fonts & colors
the mailbox view is very comfortable
Pro Very good support for Asian languages
GyazMail is a Japanese product, so it has very good support for Asian languages.
Pro Supports older versions of macOS
GyazMail is available for all versions of Mac OS X / macOS starting with version 10.1
Pro Freely selectable hard line wrap
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.
Pro Supports a big variety of character encodings
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.
Cons
Con Written for those who drink Google's Kool-Aid
It's a thinly-veiled skin over the browser interface. Why bother?
Con Can freeze very often
Con Gmail only
Mailplane 3 is a Gmail-only client. It does not support other email providers.