Automatically try the same username and password combination the first time SMTP is used
The first time you connect to an IMAP account, Evolution prompts you for its username and password to be able to fetch and receive mail.
It again prompts you for the username and password for sending when you try sending your first email.
Most people use the same mail server for sending and receiving. Evolution should be smart and just try to reuse those credentials the first time sending mail is attempted, and if that doesn't work, then it can prompt me for the credentials. Otherwise it's just inventing extra work and interruptions for the user when there could potentially be none.