GyazMail is a lightweight Cocoa-based email client, and its Japanese creator still considers it to be in a state of development. Despite this, it's fully featured and worthy of consideration if you're looking for a new email client.
In some ways, you may think GyazMail is gunning for Mail - the application is inexpensive, and its interface is simple and straightforward. However, it has a number of features that wouldn't be out of place in even the most expensive competing clients. One that the majority of users will find useful is the ability to globally toggle HTML and/or remote images in emails by way of one of GyazMail's menus.
Another great feature is the ability to explore messages that are currently sitting on the server. By default, the subject, sender, date and size are visible, and GyazMail's contextual menu enables you to label items, flag them as junk, download them or delete them. This feature could come in extremely handy for someone on holiday, temporarily stuck with dial-up, as they'd be able to leave a large message on the server until their return.
In some areas, however, GyazMail comes up short. The interface has improved since earlier versions - you can now list both 'from' and 'to' columns simultaneously in the messages pane - but it still feels immature.
For instance, the New Message window has fixed-sized fields, which means you can't easily view or edit a list of recipients (Mail expands the fields; Entourage provides a pop-up list). Also, while it makes a good job of displaying emails sent to you, it's less successful when you reply: emails are rendered in plain text, and the text of quoted emails is reflowed.
However, these are minor quirks, and as GyazMail is so regularly updated, we're sure they're likely to disappear soon.