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@aaa.com.au): Info: Connection closed: Connection reset by peer in=238 out=1892993

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