Does anyone know where the outgoing mail server is on an iPhone SE?

It has nothing to do with Apple. It is an Outlook problem. Either you changed your password and didn’t change it for both incoming and outgoing settings, or, if it never worked, you never set up the outgoing server.

All mail providers have different servers for incoming and outgoing mail. Sometimes, but not always, they have the same URL. But they always have separate settings for user name and password. And if you are setting up manually, the outgoing settings may say “optional”, but they really are not unless you are connected to the same network as the server. This is rare but possible for smartphones if you are always connected via VPN to the corporate network.

Outlook accounts are IMAP, meaning that the master copy of all email is on the server. While it will be deleted when you delete the account in wii reload when you add the account back.

yes, add the outlook account back.

Go to Settings/Passwords & Accounts. You have 2 options: The hard one is to tap the Outlook account, then the Outgoing (SMTP) server settings and correct the information (usually the user name & password). The easy way is to go to the same place, delete the Outlook account, restart your phone, and add the account back.