This error usually occurs when SMTP is not configured as part of the email’s sender identity. It is also the same as it isn’t matched with the sender’s email address. Either way, this can be fixed.
You will need to modify/edit your email settings. Here are the steps for Outlook since this one of the common email clients that experience this issue:
1. Open Outlook and click File tab.
2. Click Account settings, on the drop-down click Account Settings… again.
3. Select the email address you’re having troubles with and choose Repair.
4. Click Advanced options and tick the box for Let me repair my account manually. Click Repair button.
5. Click Outgoing mail, make sure My outgoing (SMTP) server requires authentication is ticked and Use same settings as my incoming mail server is selected.
6. Click Repair button.
Once it says your account is successfully repaired, click Done. Try to send emails again.
If you are using a different email client, you may go to the guided link on how to edit your mail server settings.