Email security settings

Ugg. I won't comment on this much other then to suggest to just NOT use Cox email. Even when everything is set up right and it "works" it will still sometimes just randomly break and there is nothing anyone can do about it, not even Cox. Atleast not on a case by case basis. 

With that said, one big part of the confusion is how SMTP works when using POP3 vs when your using IMAP5. To put simple, if possible, try to use IMAP with SSL on port 993 for inbound. AFAIK when using POP and sending mail when no mail has been recieved in a while, it does a IP check to make sure the IP sending into the SMTP server is a Cox IP. I think this system is failing and giving the false errors. If you use IMAP, you are synced to the mail servers, so I don't think it had to do the same check when sending email. 

Another part of ths issue is Cox no longer allows open relay. Another words, if your email is user@notcox.com you have to now use the SMTP server SMTP.notcox.com or what ever they are for your email service. 

Another part of the issue is if your sending from inside vs outside Cox. I have a feeling when inside Cox network, you should just use your username (no @cox.net) while when outside Cox network (Hotel, airport, etc) you might need to use user@cox.net. Think of it like having to dial the area code for a phone number ONLY if you are not in the same area code for the number already. Since the system that Cox's email is on handles other domains than Cox, when hitting it outside of Cox, it needs to know what domain you are checking the username for. 

Last, if you are still stuck, try a different email client. Some have the option of SSL/TLS for outbound encryption, and if it attemps TLS 1.3 first, it will fail, since Cox is only compatible with TLS 1.2. Other have a way to force SSL over TLS, and THAT is what you should be using on port 465. Here is a break down of the different ports and what they do for reference;