I am attempting to import a site using RaQBackup and CMu into a 550. The site and users appear but the mailbox contents don't.

During the import, I get the following error:

Creating user: matt
restoring archive for matt
ERROR undoTar: Compression not available
ERROR undoTar: Compression not available
User matt has no mail spool


Can anyone help please?

You may have gotten a bad XML on the export. Try running the cmuExport again from the source RaQ and then importing again on the target. Probably best to SCP directly between source and target, as well.