Steem Deposits And Withdrawals have been temporarily disabled On the Poloniex Exchange Because of steemit network issues,
As Said By Poloniex
steem is currently under maintenance or experiencing network issues. deposits and withdrawals will remain disabled until a solution is found,which may require an update from the steem team. any updates must be tested and audited before enabling.
Something similar is also seen on bittrex deposit and withdrawal tab, but whatever it is, we hope to have it enabled back soonest.