Time Machine error message 'not enough space for local snapshot' following upgrade to 10.13.2

Since upgrading my MacBook Air (mid 2011) to High Sierra (10.13.2) I keep receiving error messages from Time Machine 'backup could not be completed because there is not enough space for local snapshot - disk full'. Before the upgrade, the disk showed approximately 100 GB free disk space. I use an external 3TB disk with 2.5 TB free space for backup. I have followed all the usual recommendations re space management and keep getting 'disk full' messages even though the disk information shows about 50GB free space.

When I try to see what is taking up most space on the disk I am told it is iTunes (46 GB), and e-mail archives (48GB).

When I try to move the e-mail archives to the iCloud drive, Finder tells me they are already there and not on my hard disk. Also, how do I delete the local copies of iTunes music and video without losing them from my iTunes account?