Unfortunately this is a KNOWN issue with Apple Email clients, and it happens with ALL services not just Cox.
https://www.google.com/search?q=Cannot+send+mail+iphone&sca_esv=73a5863c59a470b2&rlz=1C1JSBI_enUS1075US1075&sxsrf=ACQVn0_UbUnR9q-pikt3pLQ9CbQ9WJrIHw%3A1714414718387&ei=fuQvZtKBF6fl5NoP87KVmA8&ved=0ahUKEwjSuKXChOiFAxWnMlkFHXNZBfMQ4dUDCBA&uact=5&oq=Cannot+send+mail+iphone&gs_lp=Egxnd3Mtd2l6LXNlcnAiF0Nhbm5vdCBzZW5kIG1haWwgaXBob25lMgUQABiABDIFEAAYgAQyCxAAGIAEGJECGIoFMgUQABiABDIGEAAYFhgeMgYQABgWGB4yBhAAGBYYHjIGEAAYFhgeMggQABgWGAoYHjIGEAAYFhgeSNASUOsGWLIMcAF4AJABAJgBdKABgAOqAQMzLjG4AQPIAQD4AQGYAgWgApIDwgIIEAAYgAQYsAPCAgkQABiwAxgIGB6YAwCIBgGQBgqSBwM0LjGgB8Af&sclient=gws-wiz-serp
About 122,000,000 results (0.36 seconds)
Yes, that's 122 MILLION hits for cannot send mail with Iphone. Happens with MAC computers, iphones and ipads. If you are setup as IMAP, delete the account and re-add the account, that SHOULD fix this issue.
I reported this issue back on OSX 10.5 as a beta tester, they never fixed it, and carried the issue over to their mobile devices. Good luck getting it fixed permanently, it will ONLY happen when enough people take Apple to task to get it fixed, and quit blaming email providers.