Cannot receive email

I use Apple Mail. I have never had problems with it until last night when it suddenly stopped sending & receiving emails. It also kept asking for my email password. I checked all the settings - surprisingly the outgoing port had changed to an incorrect number. I corrected it to 587. Now I can only send emails but not receive them. I called Optimum tech support who verified my settings were all correct & told me it's a Apple Mail problem & that I should contact Apple. I have a MacBook Pro (late 2013) running Yosemite 10.10.2.

Can anyone help?