A quick google indicates that the problem might be your PST has reached the max size limited allowed. You might want to check that. You might need to clean out your sent/deleted folders.
Also note that we don't know what you mean by "large files" but many mail servers won't allow a total attachment size of about 25 MB. Outlook client, based upon this quick google is 10MB. So if you need to routinely do this then consider using a file sharing service instead which works around the attachment limit and reduces the amount of space your PST needs.