It can be said of GyazMail that it is to e-mail what OmniWeb is to the Web: an alternative, integrated, paid, powerful and OS X-only client.
Since version 1.5, GyazMail supports IMAP, even if implementation is not perfect, especially regarding speed. The previous lack of this implementation can be argued to have prevented its main target (advanced users) from trying this software.
Some of GyazMail features (only differentiating features are listed):
*IMAP or local.
*Multiples addresses accounts (not exactly like Thunderbird’s identities, even if it is planned).
*Threading (different from Mail.app’s or Gmail : no grouping, but honours
*Oniguruma regular expressions support.
*previous/next in preferences (like for OmniWeb).
*Native return receipt.
*Optionally does not display signature when writing.
*Allow non-breaking space