Under your Mail Settings you usually have two sections, Incoming and Outgoing Mail Server.
The Incoming is usually POP or IMAP and the Outgoing is SMTP, if using an account like iCloud you may not see the first though.
It’s hard to be specific but if you can locate your Outgoing Server section and tap on SMTP (think you do it twice) you should come to a configuration page showing Host name, User name, Password, SSL, Authentication and Port Settings, usually in failure to send issues the “password” is missing.
Below are a couple of links to articles that should help you with regards to Mail Settings and layout configuration.
https://www.authsmtp.com/iphone-ipod-ios12/index.html
https://kb.intermedia.net/article/1377
Have a read through and see if you can identify anything missing on your device ie. like the password.
You could also go to your mail providers web site and find any relevant help articles, most post a “walk through” with regards to setup on various operating systems.
Hope some of the above helps.