When viewing a message in SpamSieve’s corpus or log, you can use the Open in External Viewer command in the Message menu to open SpamSieve’s copy of the message in Apple Mail. This lets you view the message and its attachments and search or copy its contents. You can also reply to the message or use the Message ‣ Copy Message menu command to copy it back to the mail server or into a local mailbox in Mail’s data store. For example, this can be used to restore a message that had been inadvertently deleted in Mail or lost due to a malfunction of Mail or your mail server.
If you’re using a mail client other than Apple Mail, you can set it as the default app for opening message files. Drag the message from SpamSieve to Finder to export it as a .eml file. Select the file in Finder and choose Get Info from the File menu. In the Open with section of the info window, select the desired app and click Change All….