Microsoft is often not using a standard format which is the case when you take a look at how Outlook Express saves the mails that have been send and received. Outlook Express saves the mails by default in dbx files instead of saving them in the wildly used mbx format which is being used by Thunderbird, Opera and most other email clients.

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Thunderbird saves the attachments together with the mail message which means that the mailboxes can grow rapidly if you regularly receive large attachments. This slows down Thunderbird eventually and could also pose a storage problem if you use the default profile location to store your emails.

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