26 Mail can’t send outgoing messages
Check your internet connection and mail service status, such as Apple’s at apple.co/2suZulx. In Mail > Preferences, select the affected service in Accounts, then show Server Settings. Ensure that the correct Outgoing Mail Account is selected. If that doesn’t have an impact on your outgoing messages, select Edit SMTP Server List… Review Server Settings there against recommendations. Sometimes that list gets damaged and may need to be cleared and started afresh, ensuring you have a record of settings first.
27 An email service doesn’t work
In Mail’s Window