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Pro Will probably be enough for a sizable chunk of users
Since it's the built-in Mac email client and is quite complete feature-wise, most users will not really need any other third-party applications out there. Mail.app should be enough for their needs.
Pro Can be easily extended
If the basic features inside Mail.app are not enough, it can easily be extended through plugins to add new features.
Pro Shows threaded view with snippets in middle panel
Pro Smart folders on steroids
- you can choose out of tenths of mailheaders (several hundreds including virtual headers)
- "<header> is (not) in <folder> <header>" operator which enables you to do cool filters
- each smart folder can have smart subfolders by an user definable key
Pro Easily annotate pictures or sign documents inside the client itself
Mail.app has powerful markup tools which let users add annotations to images or even sign documents they got through email without ever leaving the client.
Pro GPGMail plugin
Free encrypted mail plugin.
Con No option to snooze emails
You can not snooze emails to view later. This makes it hard to keep the inbox clean at all times since you have to read every email and choose to delete or keep them the moment they arrive to do so.
Con Missing lots of advanced features
Out of the box, Mail.app may not have as many features as some other options. This can be mitigated with plugins though. But that is not a perfect solution since third-party plugins may not be very reliable in the long run.
Con Really bad reliability
Database engine gets stuck consuming 100% of one core; crashes; a 40GB log file has been spotted.
Con Updates may break plugins
Apple is known to make API changes to Mail.app when updating which break a lot of plugins.
This can be very annoying for users whose workflow heavily relies on plugins, since they have to wait for plugin developers to make the changes needed to fix any issues brought by the update.