I revisited this issue after some days working on other systems.
I booted from a new sd card and noticed an error message in dmesg log when the sda1 disk was mounted. I ran e2fsck on sda1 disk as suggested and there were a lot of issues needing fixing - disk corrupted during the 1st April update???
Anyway all working as before and IMAP mail server back up and running again
))
If it's of interest to anyone here's the output from e2fsck:
$this->bbcode_second_pass_quote('', '[')root@alarm /]# e2fsck -v /dev/sda1
e2fsck 1.43.9 (8-Feb-2018)
/dev/sda1 contains a file system with errors, check forced.
Pass 1: Checking inodes, blocks, and sizes
Running additional passes to resolve blocks claimed by more than one inode...
Pass 1B: Rescanning for multiply-claimed blocks
Multiply-claimed block(s) in inode 6553717: 26247724--26247726
Multiply-claimed block(s) in inode 6553903: 26247724--26247726
Pass 1C: Scanning directories for inodes with multiply-claimed blocks
Pass 1D: Reconciling multiply-claimed blocks
(There are 2 inodes containing multiply-claimed blocks.)
File /var/cache/pacman/pkg/dbus-1.12.6-1-armv7h.pkg.tar.xz (inode #6553717, mod time Thu Feb 22 00:48:32 2018)
has 3 multiply-claimed block(s), shared with 1 file(s):
/var/lib/pacman/local/harfbuzz-1.7.6-2/mtree (inode #6553903, mod time Fri Mar 30 00:45:02 2018)
Clone multiply-claimed blocks<y>? yes
File /var/lib/pacman/local/harfbuzz-1.7.6-2/mtree (inode #6553903, mod time Fri Mar 30 00:45:02 2018)
has 3 multiply-claimed block(s), shared with 1 file(s):
/var/cache/pacman/pkg/dbus-1.12.6-1-armv7h.pkg.tar.xz (inode #6553717, mod time Thu Feb 22 00:48:32 2018)
Multiply-claimed blocks already reassigned or cloned.
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
Block bitmap differences: -(172233--172941) -(8552463--8552557) -(8552562--8552564) -8552573 -(8552615--8552645) -(8552651--8552697) -(8552700--8552707) -(8552709--8552734) -(8552739--8552809) -(8552817--8552959) -(8580327--8580329) -8580389 -8580397 -8580428 -(26250418--26250480) -26259609 -(26259630--26259633) -(26259673--26259677)
Fix<y>? yes
Free blocks count wrong for group #0 (3016, counted=3013).
Fix<y>? yes
Free blocks count wrong for group #5 (2326, counted=3035).
Fix<y>? yes
Free blocks count wrong for group #261 (32115, counted=32542).
Fix<y>? yes
Free blocks count wrong for group #801 (24694, counted=24767).
Fix<y>? yes
Free blocks count wrong (27075784, counted=27076990).
Fix<y>? yes
/dev/sda1: ***** FILE SYSTEM WAS MODIFIED *****
83062 inodes used (1.13%, out of 7331840)
536 non-contiguous files (0.6%)
69 non-contiguous directories (0.1%)
# of inodes with ind/dind/tind blocks: 0/0/0
Extent depth histogram: 77361/104
2227960 blocks used (7.60%, out of 29304950)
0 bad blocks
1 large file
72134 regular files
4970 directories
0 character device files
0 block device files
0 fifos
1683 links
5925 symbolic links (5565 fast symbolic links)
24 sockets
------------
84736 files