Webmail won't send, useless help page.

I think it speaks volumes that when I clicked on the Terms & Conditions for this Cox forum I got a 404 error code. Last night I was unable to send webmail because I kept getting a popup saying the mail server was unable to connect (this happens w/more & more frequency). I also heard from a friend who'd sent me an email that never arrived. No idea how many other emails I may have missed (nothing in spam). NOW when I try to send an email I keep getting error codes for 550 & CXSNDR, but Cox's error codes page offers such unhelpful "resolutions" as "Your email failed authentication checks against your sending domain's SPF, DomainKeys, or DKIM policy," which frankly tells me nothing about how to actually FIX the problem.