Mail.app (iOS and OS X)
It seems counterintuitive to recommend Sparrow to get away from Mail.app and then Mail.app to get away from Sparrow, but Mail.app has proven to be quite the workhorse. With all of the updates that came with Lion last year, Mail.app is definitely a great Mail client to use.
Also, because of Mail.app’s integration with OS X and iOS, it’s one of the only ways that you can do fun things like email to OmniFocus inbox or setup some serious rules and filters.
iCloud Mail is getting better too, but it’s a very minimal version of the desktop client and can only be used with an iCloud email account; so, that’s not very useful for people that use other email accounts and need more robust features that the desktop client offers
Postbox (Windows and OS X)
Postbox has been around for a number of years now and the creators consider it to be the best desktop Gmail client for Mac and Windows. Now with Sparrow “gone”, I’d probably have to agree. Postbox is a full featured email client that supports IMAP and POP accounts as well as Exchange (for all of us corporate types out there). Another added nicety is that Postbox supports OmniFocus and Evernote integration. Good stuff.
Postbox’s UI feels great in OS X and gives the user a vast set of features like Gmail integration, multiple account support, social network photos for your contacts, a threaded message view and more. Oh, and Postbox is only $9.95.
If you are solely a Gmail user and love the Gmail experience, then Mailplane may be the app for you. Mailplane is basically a UI wrapper around the Gmail client for OS X. Mailplane adds a lot of niceties to the web experience of Gmail like being able to get Growl notifications or even link documents on your Mac to an email. You can also have unlimited Gmail accounts in Mailplane.
Mailplane is a tad expensive ($24.95), especially considering the more feature rich Postbox is less than half that, but if you are a Gmail UI nerd, then Mailplane may work for you.
When Sparrow for iPhone came out it instantly replaced Mail.app (as much as you can replace Mail.app) on my iPhone, but before that I was using Emailganizer. The only two things I can’t stand about Emailganizer is its shoddy name and its shoddy design and UI. Other than that, Emailganizer is a super powerful app that has some great features like recognition of “context folders” (@action, @waiting, etc.), support for basically any email provider or account type you can think of, add emails as tasks to Exchange, Toodledo, OmniFocus, Things, etc., attach emails to the iOS Calendar, and more.
Emailganizer is a robust email client for your iPhone. Too bad there isn’t an iPad version yet!
Outlook for Mac
No matter what anyone says, Outlook is a pretty great email application and if you are in a medium to large company it’s pretty much standard issue email software. With the newer versions of Outlook you get threaded message views, built in calendar and tasks, extremely intelligent filters and auto responders, searching and filtering on any item and field you could imagine as well as support for any type of email account.
It isn’t cheap ($119), but Outlook for Mac is a great email tool that can definitely replace Sparrow on your Mac.
While we can’t say that Notify is a full bore email client, it does a great job of notifying you of and allowing you to preview emails and with its pro features allows you to delete, file them, or respond to them as well. Notify is more of a small app that notifies you of important emails that so you can handle them immediately, which is better than being notified of every email and being interrupted while you are working.
MailMate for Mac is a full featured IMAP email client. It supports multiple accounts and is highly “keyboard centric”, meaning that the entire app can be driven with keyboard shortcuts. There are also some great unique features like Markdown support in the email compose box as well as “Smart Mailboxes” that allow you to create smart filters and base other smart mailboxes on previous smart mailboxes. The search in MailMate is top notch as well; it’s fast and accurate.
MailMate is $29.99 but you can pick up a free trial to see if it’s a good alternative for you.
If you can stay with Sparrow just a little longer (like till the end of Summer?), then you can give Mail Pilot a spin. Mail Pilot got a lot of great press for being the company to “reimagine email”. But, more than marketing fluff, it looks like Mail Pilot might actually do it. Basically, Mail Pilot turns your email into a todo list and lets you mark items as done or even gives you some advanced controls to review them later. Mail Pilot will support all major email services, give you a single login, and even a cool smart “Autopilot” feature that lets you mark specific emails for review at a certain time (like all newsletters for a certain day of the week).