Having updated to version 6 (Marshmallow) on my Tab S2 I am unable to send emails via my BT account

My gmail account works fine and I can receive emails fmy BT account. When I send from my BT account they appear in the sent folder but no one receives them. I have checked the IMAP outgoing server settings and they are correct. I have deleted all data and re configured the BT account but it's made no difference.