no mail

I have four email accounts with cox (our internet provider). I get this message frequently when trying to send an email from our home: "The sender address xxxxxx@cox.net was rejected by the server smtp.cox.net." I have established a gmail account which works flawlessly. At least, with gmail, I can respond to emails when necessary without tearing my hair out and trying to desperately and futilely find the ports and ssl's du jour. Why can't Cox Communications fix their own mail servers to work properly??