dovecot - Oct 2016 - Connection reset by peer query

One of my user having trouble 'synchronizing' his mobile with his
mailbox

looking at logs I can see like[1]:

searching around, I found some posts suggesting index corruption??
what can or what should I do ?

can I force re-index ? delete index files ? which ones ?

using outlook on Win7, if relevant


[1]
Oct 05 11:24:32 imap(aaa at aaa.com.au): Info: Connection closed: Connection
reset by peer in=238 out=1892993

# grep aaa at aaa /var/log/dovecot.log | grep reset | wc
     10     130    1117
# grep aaa at aaa /var/log/dovecot.log.1 | grep reset | wc
     30     406    3582