Sent folder - frequent & time-consuming full resynchronisation

My local copy of the Sent folder (using eMClient version 6.0.24928.0) frequently seems to be subject to complete resynchronisation, presumably because -for some unknown reason- the local copy gets emptied. Because I have about 10 years of sent mail this takes hours to resynchronise, during which time new inbound mail dos not arrive and outbound mail gets stuck in the outbox, making the application pretty useless for usual daily email work. Please advise.