Replacing Windows Live Mail with Postbox
Microsoft has announced that starting in late June 2016 it will start upgrading Outlook.com infrastructure causing those using Windows Live Mail application to access email accounts ending in @outlook.com, @hotmail.com, @live.com, or @msn.com to look for an alternative email client. Postbox works beautifully with Windows email accounts and those who need to migrate from the Windows Live Mail can find all the instructions below.
For Users with Data Stored on the Server
The easiest way to replace the outgoing Windows Live Mail with Postbox is when all your email data is stored on the Microsoft server. If that's the case, simply follow these instructions to setup your @outlook.com, @hotmail.com, @live.com, or @msn.com account(s) in Postbox. You can also import your Outlook.com contacts into Postbox's Address Book.
Migrating Locally Stored Data from Windows Live Mail to Postbox
Via Export/Upload to Outlook.com
Microsoft recommends to those Windows Live Mail users who stored their email data locally on their computer, to move it back to Outlook.com so it can synchronize with any new email application, including Postbox. Microsoft provides instructions how to move back your emails to the server on this support page. Once you verify that all your emails are on Outlook.com, follow these instructions to setup your @outlook.com, @hotmail.com, @live.com, or @msn.com account(s) in Postbox. You can also export your locally stored contacts out of Windows Live Mail and import them into Postbox's Address Book (without the need to upload them to Outlook.com first).
Via Direct Export/Import
While the method described above is preferred, there is also a way to move your locally stored Windows Live Mail data directly to Postbox and bypass uploading them to Outlook.com. You will need to export your locally stored email data out of Windows Live Mail first. Because Microsoft uses Eml format for exports, you will need to convert the exported data into .mbox format that Postbox can accept. There are third-party utilities to do that (i.e. Eml to mbx). Alternatively, you can use a third-party add-on for Postbox to do that. Once your email data is converted into .mbox, go to File > Import... and select the .mbox files.