Cross platform. I don't use only Windows or only Macintosh. I use Windows, Mac, Linux as well as a Windows Mobile device and an iPhone. The client should run on all of these and be extremely similar yet utilize the core OS navigation/UI design for functions.
Global inbox as well as individual inboxes. Look at how Apple did this in Mail.app. By far the most powerful feature I have seen in IMAP is being able to look at all accounts or break them out.
Plugin architecture. Thunderbird and Firefox do this right. For any functions that are not included in the base package, allow other developers to add on to it.
IMAP Idle support. Why schedule a send/recieve? Allow IMAP Idle to push the messages to the inbox.
Full contacts and calendar support. Not only should the system have a best-of-class calendar and contact architecture, it should also be able to tie in to mobile devices and sync with other objects. Maybe this could all be done via that plugin architecture listed above
Simple account setup. Why do I have to go from computer to computer entering the same data. Offer a service where all of my account settings (maybe not passwords, need to figure that out) are stored online in an encrypted file. When I open the app for the first time it can ask if I want to retrieve my e-mail settings. Enter my username and password then all of my IMAP accounts, calendars and contacts begin to download to the machine. Set it all up once, deploy anywhere.
SyncML support for calendar and contacts. I should be able to not only keep my e-mail in the cloud but also my calendar and contact data. All devices should keep all calendar and contact data in sync. Think Exchange but either open source or open standards that works on anything, not just Microsoft approved devices.
Snapshot support. IMAP can be deadly if you have a POP3 client connect and delete all the messages from the server. WHAM, suddenly you have no messages in any inbox anymore on any system! Allowing the system to take daily snapshots (think SAN snapshots but on your local computer) that allow me to revert to yesterdays mailbox and re-upload all items to the server could save someone's butt some day. I should be able to turn this on or off and it needs to have server re-sync capability. This isn't just an IMAP problem by the way, it could happen in POP too.
Beautiful UI. The user interface needs to be simple, clean and elegant. Combine Mail.app with Outlook 2007 and I think you will have a winner. Or maybe think outside the box and re-invent the inbox UI.
Skype and IM/Presence Awareness integration. Both Outlook and Mail.app do this to a point, but take this to the next level. Let me know when someone o my Skype, AIM, MSN, Yahoo!, SightSpeed, or whatever contact list is online. Integrate this directly in to the address book.
Intelligent functions in the address book. Not only should I be able to add a picture of the person and see that in ever e-mail they send me, but when I select their contact in my address book I should be able to see all conversations I have had with that person as well as appointments. A person is not just a name, number and address. A person is a relationship, show me all the elements that link to that relationship on one screen.
Allow for shared calendars. I should be able to read and write to other calendars that I have permission for. See gCal for a good example of this.
Be intelligent with attachments. If the attachment is over 2MB (for example) split the e-mail in to two sections. FTP the attachment to a server and place a link to the file in the e-mail itself. Make the process completely automated so the end user doesn't have to think about it. Don't attach the whole document to the e-mail as many inboxes, spam filters and frankly the Internet in general just don't like it.
Don't be dumb about mail flags. When a message is marked for deletion don't just have a line though it, place it in the trash can. When the 'Purge' button is pressed then you can delete the message. Microsoft got this very wrong, most everyone else got it right.
In my opinion, don't even deal with POP3. This client should be focused on being the best client at IMAP and allowing for collaboration between multiple devices. POP3 does not offer this so don't even touch it.
Fast. The whole application needs to be fast. Many people live their lives in e-mail and waiting even for a couple seconds can be super annoying. Snappy factor 10.
Comment on this post
Book of the Month: Everything is Miscellaneous
Gadget of the Month: Panasonic HDC-SD1 AVCHD 3CCD Flash Memory High Definition Camcorder
Web Site of the Month: Google Docs - Used to Write Technology Evangelist Posts
Technology Evangelist Podcasts: Subscribe to Podcast Feed
Technology Evangelist Videos: Subscribe to Video Feed
Recent Comments