Unlike March's beta 10, though, this update includes more performance issue fixes than anything else. Still, it's probably a good idea to upgrade.
Most notably, memory and CPU usage have decreased. Postbox claims that indexing is three times faster in this version, compared to beta 10, and that indexing uses about 75 percent to 80 percent less memory than before. I don't have statistics to do my own comparison of beta 10 to beta 11, but I did notice empirical improvements in performance.
AOL Mail account support has been added, and a bug in importing data from Outlook Express has been fixed. There's also support for usernames with special characters, and you can now add a contact to your address book with one click from the contact panel.
Beyond those changes, the full list is made up mostly of fixing annoyances such as LDAP and IMAP connections hanging.
Postbox had problems accepting a change in my network password from the last time I used it, when I had to re-enter it four times before the program remembered it. This probably has to do with how the program talks to the Exchange server, but it was irritating nonetheless. Thunderbird, on which Postbox is based, required my new network password to be entered only once.
Postbox is a finalist for 2009's Webware 100, awarded by CNET.