While Thunderbird is a very nice mail client it can't, IMVHO, even compare feature wise with KMail. I'm not saying KMail is perfect but it has plenty of features that Thunderbird doesn't (or at least doesn't seem to) have.

Checking for New Mail in All Folders

For a start, making it check for new mail in all folders is as easy as selecting an option from a drop down list (the list is located in Tools --> Configure KMail --> Misc --> When trying to find unread messages: --> Set to "Loop in All Folders").

Checking for New Mail - A Little Gotcha

There is a little gotcha when setting up an IMAP account. By default it would seem that IMAP accounts are excluded from "Check Mail..." and also have interval checking switched off (automatic mail checking) which means you have to check for mail in them manually. Unchecking the "Exclude from Check Mail..." box will allow you to easily manually check for mail with the normal Check Mail... button in the tool bar (the box is located in Tools --> Configure KMail --> Network --> Receiving --> (Open Relavant Account with Modify...) --> Uncheck: Exclude from "Check Mail...").

While you are at it you probably also want to set up interval mail checking, the controls for which are located just below the "Exclude from Check Mail..." setting mentioned above. Use these options carefully though, I don't know, but I suspect that the reason the default settings are so is to stop your MUA from hammering the server every few minutes - be especially careful with the interval checking option as it defaults to just one minute.

Setting up Security

Using security with KMail is a doddle as well. Both the SMTP and IMAP account configuration dialogs have a button the will test the server for the best encryption system it supports and set it use that.

Attachment Warnings

The attachment warning option is another great little feature that I didn't even realize was missing from an MUA and is so simple. How many times have you said in your email "please see the attached document" and then forgotten to attach it? I know I've done that plenty of times. KMail can be set up so that it scans the email for words like attachment and prompts you if there isn't an attachment - what a great idea. The attachment setting is under Tools --> Configure KMail --> Composer --> Attachments Tab.

Folder Specific Settings

It is also possible to exclude some folders from the new mail check even when all folders are set to be checked for new mail. Simply right click on the folder and select properties and uncheck the "Include in check" box. The feature that I probably like most though is the next one down in the folder properties dialog. Under the heading "New Mail Notification" there is a check box "Ignore new mai in this folder" checking it will mean that you won't recieve new mail notifications for this folder. What bliss - this means that you can be subscribed to heavy mailing lists (like debian-user) but not get interrupted every two minutes with a new mail notification.

Folders can be set to automatically delete contents over a given age with seperate settings for mail that has been read and mail that hasn't been read. This feature is most useful on folders like sent items and spam.

There is also semi-automated mailing list handling features built in which can be activated, again, on a per-folder basis. Right click a folder and select properties. Switch to the Mailing List tab and select the Folder holds a mailing list checkbox. Press the Detect Automatically button. If the mailing list provides information about itself (and many now do) some or all of the fields in the address type drop down will be populated with information. This provides a quick way to (un)subscribe, get help and post to a mailing list. If the mailing list doesn't provide information about itself I'm not sure this feature is really worth it. To send a mail to the list right click on the folder and choose New Message to Mailing List... if the values were correctly detected it should then correctly fill in the To address without it having to be in your address book.

Other Features

Stop the press.... I think there is a feature I like more than the notification disabling. Yes you heard that correctly. I love the spell checker. As you type, any words that aren't recognized turn red and a right click on them brings up a list of optional replacements. Amazingly the optional replacements nearly always contain the correct word even with my terrible spelling. Inline / real time spell checking is an option that is desperately missing from Thunderbird.

Ok, I think that is enough gushing praise of KMail. I just can get enough of it's goodness though it is really the best bit of software I have come across in a long while. I, as I think a lot of people do, think that the KDE offerings are going to be a bit rough around the edges because they don't get the same media coverage as other projects. I happy to say that isn't the case and I will be trying out other KDE applications in the near future.