Unmounting partiton on shutdown shows failure

I have the following in /etc/fstab:

LABEL=cloudimg-rootfs   /    ext4   discard,errors=remount-ro   0 1UUID=8ff5xx..      /home           ext4    defaults        0      2 UUID=c954xx..      /home/mail      ext4    defaults        0      2 /swapfile swap swap defaults 0 0

Whenever I reboot the server, it reports a failure to unmount /home (but reboots OK).

Reading man fstab doesn't seem to offer any clues. Is it to do with the nested mount /home/mail and/or something accessing the partiton while the system is trying to unmount it (eg dovecot)? If so, how can I ensure that everyting has finished using it before it's shut down?

lsblk

xvda    202:0    0    55G  0 disk └─xvda1 202:1    0    55G  0 part /xvdb    202:16   0    20T  0 disk ├─xvdb1 202:17   0     5T  0 part /home└─xvdb2 202:18   0    15T  0 part /home/mailxvdd    202:48   0     5G  1 disk └─xvdd1 202:49   0     5G  1 part