I had the same problem and just recently got it fixed.This is what the technician advised me:
Go to your apple icon and scroll down to system preferences. Then dependant on what machine you have it is different for my imac and mac book pro click on either air port/wifi then the advanced tab in the bottom right hand corner. Then click on the DNS tab. Hit the + sign on the left hand side and type in 188.8.131.52 then hit the + sign again on the left hand side it will create a new line underneathe and type 184.108.40.206 then hit okay on the bottom right and then hit apply, again on the bottom right. Then send a test email to check but it should be fixed.
Had to share this. Its been driving me mad for months.