Q: Mac Mail will only work with one server...how do I get it to work with both?

I am having so much trouble with Mac Mail.  I have two servers iCloud and Cox (my isp).  Cox is constantly offline and will not work.  It actually goes off and on by itself....I have no idea what causes it.  iCloud generally works.  I would like to use both but I cannot figure out how to make Cox work.  Cox gave me the port number which they said would be right....however, nothing worked.  Apple said not to use Cox's port number to use theirs which is 587.  It sometimes will work, just briefly, before Cox goes offline and doesn't work at all.  It will do nothing with Cox's number (425).  For iCloud I use 993 which seems to be fine most of the time.   

I know others are having many problems with Mail, too....I just wonder if anyone has a fix yet.  And, if Cox will not stay on line which apparently is not Cox's fault, but is in my computer (Yosemite?) should I continue working with Apple?  My computer has another year on its warranty.....it seems to me I must get it working before the warranty is up.  Or, does it matter as long as iCloud functions ok?  I have no problems with mail on my iPad....only my iMac.