Mail.app and all current mail clients have two separate connections. Receive, and transmit. A receive path via POP or IMAP that fetches messages from the mail server, and a send path via SMTP that sends outbound mail to the mail server. The two paths are quite separate. Often on completely different mail servers, too. Both the send and receive paths have their own user and password specifications, a port setting (or the default port), and a server, and whether or not SSL/TLS network encryption is in use (SSL/TLS is usually enabled) on the path.
The receive path (POP or IMAP) is the one you can immediately see and access in Mail.app Mail > Preferences > Accounts. If that's working, you can ignore that path.
The transmit (SMTP send) path is only accessable and only visible via the Edit SMTP Server List... selection of the Outgoing Mail Server (SMTP) selector, and you'll then get to a second set of accounts and settings displays; these for the send path. Something in those settings is incorrect, for your particular ISP.
Also use Connection Doctor (Mail.app Window > Connection Doctor) and the documentation available from your particular ISP to determine which of the SMTP server path settings are incorrect. If you can't determine what's happened from the ISP configuration requirements and the Connection Doctor output — open the drawer and ask for details within Connection Doctor — then expurgate any personally-identifying information from the detailed output and post it here. If you do post those details, please expurgate just your own details. Please leave the ISP and the mail server host name unexpurgated.