"The messaging interface has returned an unknown error. If the problem persist, restart Outlook." it is one of the most common error received by outlook users and this usually states of corruption issue with the concerned file, but in cases other than, oversized pst file may also gives rise to the issue. This is since you are not only unable to add files to it, but the files could not even be deleted. First try and scan the pst with scanpst.exe, if it does not help try using pst splitter tool.