I did wonder if it was caused by SpamBot malware on your network causing your home IP address to be blacklisted. That would stop you sending but still allow you to receive emails. However, you have already said that the problem remains when you switch to mobile data so it's not that thank goodness.
You mentioned earlier "This only started happening after the iOS update to version 13." and it does seem there have been reports of problems with email which have been linked back to iOS 13.
I am not an iPhone user so cannot offer much practical advice I'm afraid. However, would it be worth (or indeed possible) to try rolling back to the previous iOS version and seeing if the problem still persists?
******************************* I am just another Virgin Media customer. If someone posts a useful reply you can say thanks by clicking on the thumbs up sign in their post. If someone posts a message that solves your problem it helps everyone if you mark their post as a Helpful Answer
I can do everything from webmail and from outlook on my pc. Only issue is sending mail on my iPhone while on both mobile data and WiFi. I have tried to take my phone back to the previous os12 but os12 is now not signed by apple to recover my phone. I’m going to update to os14 tonight to see if that works.
I have tried forgetting email addresses on my phone. I have also tried on an android phone but still will not send emails. It has now gone that I can not send emails from anywhere other than webmail on 2 of my email accounts. My wife and sons have no issues sending emails so I know my wan IP address is not blocked it has to do with my email account settings. Either my password is not updating on the SMTP server or my account has configuration errors.
I get an error message saying smtp server username or password is incorrect and the just stay in my outbox. If I delete my email account on my phone and set it up with my wife's email it lets me send and receive email from her account. So it's not the phone or the app. It has something to do with the server config.