I've been searching for solution as many of you but apparently there is no solution. COX support was useless.
I can receive my emails through my iPhone X and no issues with incoming or outgoing emails with my imac/macbooks at home or via webmail.
I just can't send emails through my iPhone,even after verifying the settings, deleting and re-adding the email accounts, or changing the passwords.
I also have no problem receiving and sending emails for my work emails on my iPhone so it's definitely something to do with the Cox email server.
I hope there will be resolution soon.
Additional Note:
After I posted the above, I reread some of your posts and I just concluded I have the same issue with Steve A. I can send emails through my iPhone once I disconnect from my work WiFi. Not sure why this is the case now since I also didn't have any issues prior to April 2019.
I'm not 100% certain but I think my issue when I'm at work on wifi is that my company doesn't provide a Reverse DNS lookup for when the Cox SMTP servers are trying to accept my SMTP connection and authenticate who I am. Just like you.... when I'm at home on WIFI or just on AT&T cell service everything works OK. I've already asked our firewall people here at work to see if packets are flowing from our network out and back to/from SMTP.COX.NET port 465 and the packets are going back and forth and not being blocked. BUT....apparently if Cox can't get a good result from the reverse dns lookup then they reject your connection. If you look in other threads you'll see this mentioned by Cox support. Look in this discussion:
https://forums.cox.com/forum_home/internet_forum/f/internet-forum/23071/blocked-ip/88476#88476
I've asked my company if they can allow the reserve dns lookup and I was told No that because of security reasons they won't. So....if I need to send an email from my iPhone when I'm at work I then have to turn off WIFI to go back to AT&T wireless and then send the email.
