Verify an email account’s outgoing server in Mail on Mac
If you see an alert that your message can’t be sent using a specific outgoing mail server, it’s possible that you’re not using the required server for the email account, or the server may be configured incorrectly in Mail preferences.
In the Mail app on your Mac, choose Mail > Preferences, click Accounts, then select an account.
Click Server Settings, click the outgoing Account pop-up menu, then choose Edit SMTP Server List.
Review information for the server; change settings as directed by your email account provider.
Don’t remove a server that’s in use by an account unless you’re ready to add a different server for the account.
Test any changes you made by sending yourself a message.
When you choose a different server for an account, all messages you send from the account use that server until the network status changes. The status can change when you move from one network to another (say, from work to home), or if your Mac goes to sleep and wakes up in a different network environment.