Unable to send emails

I am running High Sierra 10.13.2 on an iMac. I am running four email accounts in Mail, two of which are with my ISP (Cox). Mail will remember the outgoing server settings for one of the accounts, but not the other. I have not been able to get more than one email account to send mail. Prior to the last OS upgrade, I had no problems receiving and sending emails with both accounts. I have given each a different description and outgoing mail server account name. I have tried letting mail manage connection settings and inputting port and authentication settings. I have tried deleting and adding the email accounts back in. All without success. The account works fine using ISP Webmail. When ever I enter user name and password for the outgoing mail server, the account switches to "None", or drops the User Name and Password.