Been using GyazMail for a long time now, and just get more impressed as time goes on... the thing just rarely crashes. Development seems to have slowed a bit, with some major features not in that I'd really like to see (GPG, iMap, spam filtering) but you can't knock the quality of what is there.
There are a lot of niceties I'd really like to have in it, which I've included below. The wide-screen link goes to a page where I show some screen shots of gyazmail that I've messed with the nib file on to give it a wide screen mode.
A delete thread command. This really speeds up going through larger lists, especially with the preview pane. Just make it command+option+delete or something.
Don't update status icon with a delete icon whenever you remove a file. I guess if your machine is super slow it might be nice, but really, when you're deleting 100 messages having to wait for it to refresh is a drag. Apple did this with their finder recently for a nice speed boost.
An empty all trashes command would be nice, as having to select all your mailboxes before you run the command is a drag.
Allow the damn "end" & "home" keys to work in a message. For some reason, they dont seem to.
Double clicking a seperator bar should collapse the side/bottom view, while remembering the current sizes when you expand again.
Why show the horizontal scroll bar at all if nothing needs to scroll? Gyaz does this, both in the mailbox view and the folder view, for wasted space.
A "hide/show preview pane" button for the toolbar would be nice.
An undo command for more actions. IE, when you delete an item accidently, being able to hit "undo" and have it moved back would be a lot nicer than having to go to the trash, fish it out, and then moving it back.
It'd be nice if the preview pane was smarter, and remembered its size depending on the folder. IE, in some mailboxes i don't get that many messages, so I'd want the preview pane to be larger. In others, I'd want it smaller to better keep track of threads.
Ability to select "search within all folders" (or more customizable options) would be awesome without having to select all the folders.
Please show quoting and formatting information while you're composing an email. If it is going to wrap, show where it will wrap! Showing colors for quoting makes things so much easier to follow and respond to while composing a message. I've requested this from the author, but never heard back.
I don't understand why "new message" and is under the "Message" menu item instead of the "File" icon. Not a big deal, but I always have to look twice for it as it's not where I'd expect.
Wide screen support!
Alternating colored rows for threads. Very helpful for scannability. I suppose you could also give alternating rows as an option for individual messages too, ala iTunes.
I'd wish the grid would just seperate out the messages themselves, not also the subjects and attachments... IE, horizontal lines, but not vertical lines.
Add a global "take all offline" menu item, or key combo. Because really with how Gyaz is setup, if I don't want a bunch of error messages when I can't connect coming up I have to take it offline, which means selecting all the mailboxes. No reason why there can't be a global command.
Option not to show messages in the trash as read, as it reduces scannability when looking for folders with unread mailboxes.
Option to only check for new mail when you have an active internet connection, so I am not bombarded with can't connect messages.
Consider throwing the can't connect messages into a single window, download-manager style. Having to hunt for them (I have a lot of mailboxes) is a drag.
A global search option from within search mode. Having to select individual mailboxes is a chore when I don't know what folder or mailbox the message was in...
If the mailbox with unread messages is not collapsed, don't highlight the mailbox itself as unread, it reduces scannability when looking for folders with unread messages.
Add a "collapse all" and "expand all" option for the folder view.
Better integration with the Address Book: especially in a wide screen mode, there's more space to pretty things up and perhaps snatch picture info, etc.