I have five email addresses, four business ones and one personal address. If I don’t set a default account, each time I write a new email it's sent from one of my business addresses. If I want to send the mail from my personal address I have to remember to choose the correct personal account from a drop-down box.
Most of my email correspondence is business related, so I want my Apple World Today email address to be my default email account. Thankfully, that’s easy with macOS High Sierra.
Open Mail, then click Mail on the menu bar, then Preferences, then Accounts. Highlight the account you want to be the default, then under Outgoing Mail Server Select “Use only this server.”
If you want to use another mail server to send a message, in the New Message compose window, below To, Cc, Subject is the From line where you can select a different server from which to send your mail.